DeSantis is much more anti gay than Trump, which is probably also a factor as most under 30s, even most under 30 Republicans, have at least one non straight friend.
I don't think that has much to do with it. I think younger voters are less informed and are probably less likely to have heard of DeSantis, and young Republicans tend to care more about owning the libs and other empty crap than the policy positions that conservatives like. Seriously, look at successful conservative media aimed at people under the age of about 45 and
all of it (yes, 100% of it) revolves around looking at crazy liberals and talking about how offended they are. So they're going to pick the candidate that they feel offends liberals the most. Their support of DeSantis may change once the liberal propaganda machine starts demonizing him more and we start seeing pundits crying about how mean DeSantis is.