They failed primarily for pricing reasons. They could make a low power CPU competitive with ARM (especially back then when Intel had the state of the art process), but then they wanted to charge a premium for it being x86 and the OEMs turned up their nose at that.
Samsung still has a fairly competitive process and could make x86 CPUs to put in their own tablets and laptops without having the OEM and Intel get into a fight about margins if they're the same company. And with the largest maker of Android devices putting x86 CPUs into them, you get an ecosystem built around it that you wouldn't when nobody is using them to begin with because Intel refuses to price competitively with ARM.
