What are the reasons for the lack of knowledge about gnostics and their beliefs among the general public?

Because Gnosticism was really a set of religious ideas that died out between the first and second century.

And it died out for a good reason:

When mankind tries to make to heaven of his own accord, he usually tries to do it one of two ways:

Some form of “spiritual-ness” that excludes or denies God in some way. This is the non-rational approach.

Some form of intellectual exercise, or rational path to God. This usually excludes or denies the spiritual.

And of course man can make variants off of these two basic things by combining them in strange ways. Usually ending up with superstitions and / or man made rules to follow to try to get you to heaven.

In the case of Gnosticism it is a nearly completely an intellectual exercise, as it is based around knowledge. And it was a good thing it fell by the wayside.

Why…

Because Jesus made it very clear: He is the Way the Truth and the Life. And no man gets to the Father except through Him (John 14:6). Paul goes on to nail this down even further in case people missed it. You can’t earn your way into heaven. You are not good enough, and nothing you do will ever be good enough. We are saved by Grace through Faith (Ephesians 2:8-9).

Why did Paul tell us that? Because mankind always thinks he can. Almost the whole Bible is a lesson in showing us we can’t. Under any circumstance.

Being innocent and don’t eat the fruit? Nope. Kicked out of Eden.

Have the knowledge and just behave? Nope. Flood.

Behave (implied) and just spread out and fill the earth. Nope. Babel.

Create a pedigreed race of priests and just follow the rules (Law). Nope. Failed so many times I’m not writing them down here.

Then Jesus shows up, fulfills the Law and throws mankind a life ring. Many take advantage of this. Others still keep trying to get to heaven on their own. Follow Jesus. He will get you there (John 3:1–15, Acts 2:38)

Hope this helps :)