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He could not have been forgiven because he did not give Jesus the chance to forgive him. He regretted what he did, threw back the 30 pieces of silver "Saying: I have sinned in betraying innocent blood" then went and hung himself - which is a mortal sin. 

Don't forget, Our Lord said: "The Son of man indeed goeth, as it is written of him: but woe to that man by whom the Son of man shall be betrayed: it were better for him, if that man had not been born." [Matthew 26:24]

So when Our Lord said that it would have been better had he never been born would be false if Judas had been saved, and therefore Judas is lost.

Simply regretting one's actions is not the same as repentance.  Instead of weeping and converting/changing after St. Peter did with his betrayal, Judas despaired.  When someone despairs, there's a total self-absorption or self-focus there, where you're upset exclusively because of the impact the sin had on you, where you have the guilt of the crime ... rather than repenting due to the consequences of your actions.

Judas could have avoided despair had he recalled that Our Lord said that the betrayal and His crucifixion and death had to happen ... something that even the other Apostles appear to have either forgotten or not believed or just "filtered out" due to their not wanting to believe it (cognitive dissonance).

Bottom line is that regret by itself does not equate to repentance, since true reptentance depends on the motivation.  Why did he "regret" it?  Could it just have been because he'd certainly be kicked out by the Apostles from their fellowship and his "grift" (of stealing) would have been over?  Or because his reputation would be destroyed?  We don't know.  Now, after Our Lord's Redemption, we can have a selfish motivation (imperfect contrition) where we do regret the consequences the sin has on our souls, and that suffices in the Sacrament of Confession due to Our Lord's Mercy, because He already supplied the pefect contrition and satisfaction for our sins.

But Judas appears to have had an at-best imperfect contrition for his actions (self-focused), which as stated outside the Sacrament of Confession, does not restore someone to a state of grace, but then also despaired (again due to the self-focus).

So, the Gospels also mention that Judas, when he objected to Mary Magdalene's anointing of Our Lord, wasn't actually concerned about the poor, but he had been embezzling from their common funds, so his corruption goes way back.  We don't know why.  Did he just not have faith in Our Lord or lose it along the way?  Or was he just overcome with greed?  If the latter, that also factors in, since, as the Council of Trent teaches, there can be no justification without faith ... so if Judas had no faith, he could not have been saved anyway.  Without faith, you could regret any bad actions of yours all you want, for natural reasons, but without that regret being animated by supernatural faith, there can be no forgiveness.  Recall that NO AMOUNT OF REPENTANCE on our part suffices for forgiveness (per St. Anselm in "Cur Deus Homo") ... but only because Our Lord's Redemption sufficed, and we can only have the fruits of His Redemption communicated to our souls if we have faith in Our Lord (and then of course supernatural charity, etc.)

If Judas had had faith, and at least imperfect contrition, he could have gone to the other Apostles and been absolved of his sin (since Our Lord had given them the power to forgive sins) ... but he may not have had any faith at all, and he did not seek forgiveness and therefore his imperfect contritiont could not have sufficed to restore him to a state of justification.