Nostr has no economic spam countering mechanism on the protocol level (the protocol is inherently free, like SMTP) which means it's up to relays to fight spam and that's an inevitable centralizing force. 
Fortunately the protocol could yet be overhauled to fix this using something like HTTP 402 (i.e. make everything paid). 
Till then the network is zombie walking towards an inevitable censorship reality once it's big enough and spam is burdensome enough.