The reason is that "why aren't many people running Bluesky instances" is a loaded question — it presupposes that's something you should expect. But it's like asking "why aren't many people running Google". It's expensive to store billions of posts!
This is usually compared to Mastodon, but it's not apples to apples since "running a Mastodon instance" always mean a tiny partial view of the network. You can do a partial view of atproto too for the same cost, it's just not very interesting because what's the point? Every app can "see" the entire network (unlike in ActivityPub) so there's a lot less motivation to create these fragmented views into it.
I've already linked elsewhere, but I've written an article on precisely this topic, curious to see if this can clarify it for you: https://overreacted.io/there-are-no-instances-in-atproto/#so...