I think it is a combination of issues rather than any one thing. Technical issues, UX issues, limited range of content and content discoverability issues. 

I think the biggest issue though is that most “normal” users don’t really perceive right now that there is a need for a decentralised, censorship resistant protocol. They have no understanding what that actually means and don’t really give much thought about the issues with the centralised platforms. They are using Facebook just to share photos of their kids or watching and posting stupid videos on TikTok. They have never had issues with censorship so why would they even think about using Nostr?