It's most likely because the nativity of St. John was intimately tied to the Nativity of Our Lord (cf. the Visitation).  With most saints, the nativity itself wasn't one of the defining moments of their lives (other than by virtue of the fact that it's when their road to sainthood started).

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St. John the Baptist has the SINGULAR honor of being the only saint (other than Our Lord and our Lady, of course) whose birthday is celebrated in the traditional Catholic Church Calendar.  

This makes a kind of "trinity" of birthdays, for all time.

Can anyone explain the reason there are only these three, and not more?  We have a lot of saints to choose from, you know.  

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If I understand the question correctly it is because the three are the only absolute certain three that came out of the womb without Original Sin.

Quote from: Neil Obstat

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St. John the Baptist has the SINGULAR honor of being the only saint (other than Our Lord and our Lady, of course) whose birthday is celebrated in the traditional Catholic Church Calendar.  

This makes a kind of "trinity" of birthdays, for all time.

Can anyone explain the reason there are only these three, and not more?  We have a lot of saints to choose from, you know.  

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If I understand the question correctly it is because the three are the only absolute certain three that came out of the womb without Original Sin.

Precisely correct!

Holy Mother Church celebrates the three (only) nativities spotless as if newly baptized.  Only Our Lord (God Himself is not subject to corruption in any way), our Lady (by the "singular privilege" of her Immaculate Conception), and by St. John's miraculous reception by the intercession of the Blessed Virgin Mary have birthdays recognized on the Church calendar, for this very reason:  they were free from original sin before birth.  

Now, it's possible for any child to be free from original sin before birth, if baptized inside the womb of its mother.  I knew an anesthesiologist who had baptized some such children who had been on the verge of death yet unborn, so it can happen, but the three cases above were entirely by God's providence, that is, entirely due to His grace, and without the action of man (by pouring water over the in-utero baby's head and speaking the words of holy Baptism).

Notice how personally involved Our Lady is in all three cases, which refutes the Protestant heresy that she has no power of mediation between God and man.  When one such heretic told me that, I said that nowhere does the Bible say there is no MEDIATRIX between God and man.  He stared at me with his mouth open and then he ran away.   In fact, he was with a whole congregation of Protestants and his "pastor" glared at me while ordering all his "flock" to pack up their stuff and leave the public park they were enjoying until I came along.  It would seem he was afraid he would lose some members if they had a chance to speak with me.

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The fact is, that St. John received this singular privilege, just as Our Lady received the singular privilege of her Immaculate Conception.  There never was, nor will there ever be someone else immaculately conceived.

St. John the Baptist was immaculately conceived? I never knew this!

Far as I knew, theologians only speculate that St. John the Baptist was born without Original Sin, but not immaculately conceived. I always thought that no one really knows definitively, and that it is only speculated that when Elizabeth heard the salutation of Mary, the infant leaped in her womb and his Original Sin was removed at that instant.

Can you post a link or something?  

PRAYER TO ST. JOHN THE BAPTIST AS YOUR PATRON SAINT

Saint John the Baptist, whom I have chosen as my special patron, pray for me that I, too, may one day glorify the Blessed Trinity in heaven. Obtain for me your lively faith, that I may consider all persons, things, and events in the light of almighty God.
 Pray, that I may be generous in making sacrifices of temporal things to promote my eternal interests, as you so wisely did.  Set me on fire with a love for Jesus, that I may thirst for His sacraments and burn with zeal for the spread of His kingdom. By your powerful intercession, help me in the performance of my duties to God, myself and all the world. Win for me the virtue of purity and a great confidence in the Blessed Virgin.
Protect me this day, and every day of my life. Keep me from mortal sin.
Obtain for me the grace of a happy death.  Amen.
                                       
A MOTHER'S PRAYER TO ST. JOHN THE BAPTIST

St. John, child of saintly parents, child of prayer and grace, gift of heaven, forerunner of Jesus Christ, preacher of penance, preparer of the way of the Lord! You are the model of all those that, with an upright heart, seek Jesus and desire to belong to Him. Although innocent and pure as an angel, you performed during your whole life the strictest penance. Scanty and poor was your nourishment. You abstained entirely from intoxicating beverages. You perseveringly shunned all occasions and dangers of falling into sin, and spent your time in converse with God, in meditation and prayer. You daily increased in wisdom, courage, and strength of soul, and daily exercised yourself in the practice of those high virtues demanded of you by your vocation. You, by the holiest actions, proved the truth of your enlightened teaching, your glowing admonitions, and your menacing warnings. You aroused hearts to faith in Jesus Christ, inflamed them with His love, and incited them to His imitation.
O holy St. John, take my children, take me, and all my dear ones under your protection and guidance! Show Jesus to our hearts. Teach us to believe firmly in Him, to repose unshaken confidence in Him, and to love Him with our whole soul! Be, finally, our mediator with Jesus Christ. Oh, obtain for us the pardon of our sins, worthy fruits of penance, entire satisfaction for all the offenses committed against God, faithful imitation of your life, strict curbing of our evil fancies and perverse inclinations, an ardent desire for heavenly riches, perseverance in grace, fervent imitation of Jesus, the patient bearing of our cross, and in the hour of death your blessed company for all eternity. Obtain this for us, O well-beloved of our Divine Savior! Obtain it by your high rank in the Kingdom of God, by your superabundant merits, by the love you bear our souls, and by the ardent desire you have to see us all enclosed in the Heart of Jesus, there to be preserved, guarded, and saved forever! Amen.