Marriage is a far more attractive proposition than it used to be, and therefore, if there is a religious vocation, it gets drowned out by the various things that are perceived to be appealing about modern marriage. (I said "if there is..." --- it's possible, in our faithless age, that Almighty God is simply not calling these people. I won't dogmatize either way. That's out of my wheelhouse.)
Before the modern era, marriage for a Catholic meant entering into a union where, even with use of the "rhythm method", more children would come than would be desired or convenient. The man would have to work hard all of his life, possibly even dying young due to work-related injury or illness (my own grandfather, who was not Catholic, died young for that very reason, and it is highly unlikely that my grandparents "used anything"). The woman would have a very hard life of raising all those children, having more pregnancies than she would have preferred, and would have to put massive amounts of effort into maintaining a home and large family. Moreover, divorce was impossible --- if the marriage went sour, you were stuck. Looking at it this way --- and, as I mentioned in another recent posting, you then get this blubbering from "Novus Ordo Puritans" that "oh, no, no, a vocation has to be pure, it has to be solely from the highest motives" --- the priesthood or religious life doesn't look so bad. You get an education, you get a career, you have lifetime job security, your work isn't herculean, really, aside from having to give up sɛҳuąƖ relations --- let's just call it like it is --- it's really a pretty sweet, fairly easy life. And if those sɛҳuąƖ relations --- again, let's just call it like it is --- are going to get you in all that trouble, make your life so difficult, maybe it's easier just to turn off that aspect of your life, and be done with it.
But now you have an entirely different marriage paradigm. You get to have one, two, three kids tops, basically, you can order children like you order pizzas, you have them when you want to and not a moment sooner, and when you're done, one or the other "gets fixed", says "I've been blessed, I'm done having kids", and it's sex on demand for life without consequences. One big party! And if the marriage goes south? Easy peasy, get the divorce, start "dating" again (in the secular world and the world of cafeteria religionists, premarital chastity is unknown, it's pretty much sex by the third date, fifth or sixth "if you have good morals"), usually end up moving in with one another, lather, rinse, repeat. Marriage ends up being pretty much one big date and one endless good time, with a fun-sized family, or if you don't want kids at all, hey, that's okay too, just enjoy one another. You can go to Mass (the Novus Ordo), receive communion, either never go to confession, or go and don't bring up the contraception thing --- you'll probably never be asked --- fun, fun, fun, rockin' in the free world! When you can have a life like that, then a priestly or religious vocation is going to be absolutely, positively, the last thing you would ever think about.
I don't think it's any more complicated than that.