They have over 1.8 million users currently, or do you mean PDSes specifically? Federation is in open beta on a test network, you can try it out today if you'd like.
There are two usual strategies for growing: 1) low cost, organic and slow or 2) high cost, throw a lot of money at advertisement, saturate all media, grow quickly or bust.
The exceptions are those rare products that despite a low cost marketing sell themselves so well that their organic growth is fast and in a few months everybody use them.
Maybe Bluesky don't have the money to advertise or is not compelling enough. As one data point: I know about Mastodon but I think that I learned about Bluesky only today. I went to their site and there is nothing to explain how it works except that it's some social thing. I learned more by reading the comments here. Apparently it's being marketed at a very low cost.
