I think your question might be opinion-based and therefore off-topic on this site, but I'll still try to give a helpful answer.
Bitcoin maximalists believe that bitcoin is the only cryptocurrency worth holding, studying and/or building upon. That belief comes from any of these and/or related claims (with varying degrees of verifiability):
- Bitcoin is the only cryptocurrency that is truly decentralized. (Its creator is unknown and has not been involved with the project for many years, and there is no central authority saying what the protocol changes should be.)
- Bitcoin is the only cryptocurrency that is distributed fairly. (There was no pre-sale for insiders, no pre-mine, everyone had the opportunity to mine right from the start.)
- Any claimed improvements brought by other cryptocurrencies can be implemented in bitcoin as well if they are shown to be useful.
- Bitcoin has the most proof of work going into it, making it the "most secure" chain.