Really a lot of work. Software compatibility and availability will make or break a processor. Unless you own the entire stack and are willing to deal with the struggles of a custom ISA, nobody wants to make a new one - the benefits probably aren't that great.
This can't be overstated for a cloud provider, especially one like AWS that wants to run everything for everyone. ARM has good OS and compiler support, and if you want people to move some workloads off x86 to another architecture, you're gonna want that migration to be as painless as possible.
Really a lot of work. Software compatibility and availability will make or break a processor. Unless you own the entire stack and are willing to deal with the struggles of a custom ISA, nobody wants to make a new one - the benefits probably aren't that great.