Probably no one will believe this but it's just because Intel didn't put much effort into the phone Atom chips. They could have been really good.

They're also not that different because the latest x86 extension turns it into arm64.

https://www.intel.com/content/www/us/en/developer/articles/t...

Fundamental difference when it comes to performance.

In terms of implementations, obviously arch matters a lot in terms of the "established" software ecosystems, but if you're writing some software and need to hit a performance target no arch has a particular advantage solely because of its architectural implementation.

x86, ARM, etc. are huge ecosystems and things are orders of magnitudes more complex than we use those letters vs you use these letters for ISA.

The answer for your question can be: because somebody didn't put enough effort / attention to make it work

Is it true? I dont know, but can be