Because it’s an open Instruction Set Architecture.
Many different companies used to design their own CPU IS architectures in the past like (MIPS, AVR, PIC, …) and of course the most popular ARM. Downside of this is that the software and ecosystems between these architectures are not compatible. Effort wasted in porting a library to one architecture cannot be always reused for another.
Recently we see a lot of companies adopting RiscV, and there is a big collaboration between them to ratify the specification and provide software support. This will in turn accelerate the development, and software and hardware support will hopefully overtake ARM in the future.