Because x86 was technologically mature in the 90s (superscalar, OoO, vector extensions, …) and basically hit its frequency ceiling in the mid-aughts (modern Core-series finally have frequencies beyond what P4 reached back in 2005~2006 but it took a long time).

ARM was nowhere near there (for it had no need to) before modern smartphones. ARM in 2007 was single-issue in-order single-core topping out at ~600MHz.