I would say UX, the maturity of most clients, a bit confusing to understand what’s special with Nostr (concept behind relays NIP’s) if you aren't familiar with the technical aspects. 

Also big one is the network effect. 

Another one is that a lot of people keep promoting Bitcoin. I can understand it being a bit off-putting if you aren't into crypto or understand what it is. For people that are into other aspects of life and want to find like-minded people.