A lot of it is because it is newer, and the designers have learned from previous architectures. It is a relatively clean and straightforward instruction set, designed to be easily and efficiently implemented.
There's not anything that is super crazy revolutionary, in contrast to the (still vaporware) Mill CPU architecture.
> Is it likely to evolve faster because it is open or more modern?
They have a good extension mechanism that allows relatively clean additions to the instruction set. Some of the recent ones like the vector extension aren't finalized yet. Anyone can propose their own extension. Historically, ARM might work with their most important customers to implement an extension, but good luck getting their attention if you're not already paying them millions per year.