I guess one of them is that a running witness that doesn't agree with you and has more voting weight can downvote and hide all your posts/ruin your reputation out of spite. d:^)
Jokes aside, I think there's multiple reasons:
I believe overall, Steem/Steemit doesn't have enough marketing to make itself really known. While most other coins quickly gather awareness over various forms of social media and general word of mouth (or word of... text?)
As an investment standpoint, I don't think many people that came into crypto over the past year are looking for long-term investments, but more short-term holds and maximum gains. After all, that's what they were introduced to, it's what they expect.
The other thing is certainly money. Steem is already at a decent price (I've bought 1000+ recently), but it doesn't really feel like a deal. If you look at many of the smaller coins that really blew up during the bull market, they had incredibly low prices. Many of the cheaper ones blew up for one reason: they were low-satoshi, high-circulation coins. Steem is already somewhat established.
People tend to come to Steemit to make money, so the premise of investing into Steem probably doesn't make much sense if you're a struggling content creator. Chances are you don't even have the capital to do so, and this is coming from someone that was a struggling freelance writer for several years.
I'll add a few more reasons to this comment if I think of more.