why doesn't everyone become a Buddhist to attain supreme bliss?
I think that a "true" Christian may have beliefs which (may) help them to find some peace or solace, and which have some similarities at least in their effect (if not in their form) to Buddhist beliefs. I guess for example, faith that "It's God's will" or "This is part of God's plan", and the "Lord's Prayer" that, "Let thy will (not my will) be done", might help to alleviate a personal craving for something to be other than it is (and therefore, alleviate that type of dukkha). Similarly the virtue of Charity (which is Christianity's "greatest" virtue, and the second of the two "great commandments") remind me of some things which the Dalai Lama has written, about caring for other people to avoid being trapped in your own "self".
I know less about Hinduism; but a while ago I asked about this story which I half remembered, Self like a diamond, because I assumed its moral was Buddhist: I recently discovered that story is from a Shivaite tradition ... which I haven't been taught, but of which I get the impression that the principal doctrine, attitude, or action seems to be something like, "Everything is God, let me lose my self in order to find or be with God" ... so again there's some kind of non-self doctrine happening there.
I'm not trying to say that these other traditions are as good as Buddhism, nor the same as Buddhism ... but perhaps they address or attempt to address some of the same human frailties ... and people's needs are at least partly addressed by the religion they have.
Another answer to your question is that there are historical reasons. The Buddhadharma has existed in the West for only about the last 100 years or so ... if you learned any religion from your parents, it probably wasn't Buddhism.
Sometimes people who have any existing religion might be averse to learning or practicing a new one: Islam may penalize apostates; Judaism (and Christianity) have the first commandment; see also Meditating and including buddhist philosophy in your life without abandoning previous religious beliefs?
Then again, maybe your premise ("why doesn't everyone become a Buddhist") is wrong ... maybe everyone does become Buddhist! I discovered that "zen" has become a well-known, everyday or "current" adjective in France ... a synonym for cool or calm, detached, unstressed. So everyone has some idea of what it means, what its values are. People who are interested can find more ... there are many centres (e.g. in Paris), there's Plum Village, there are books in a large local book store, etc.
Also, as I said in How to explain what Buddhism is?, I've found that not everyone seems to feel the need for what Buddhism has to offer ... and/or that Buddhism isn't sufficiently well taught, well explained to people ... so that's another reason that not everyone is interested. I won't say "there aren't enough good Buddhist role models for people emulate" because maybe that's not true, however maybe those good role models (who other people might meet every day in society) aren't people who preach, or teach that Buddhism is essential -- How are Buddhists supposed to spread Buddhism?
Finally the answer I accepted to that question suggests that there are different forms of Buddhism ("each branch has a slightly different take on what the problem is that Buddhism is meant to solve"). If you think there's "one true form" of Buddhism which results in "getting Nirvana", you might note that not even all "Buddhists" will become that type of Buddhist.