I think that's the point, though: why didn't they stick? Was it a price/performance/power issue, which would point to ARM being somehow "better" on those dimensions, or was it something incidental, like production availability, or even just perhaps a lack of desire at Intel to aggressively pursue that market?

At least some of the attempts in question had, ahem, interesting ideas for how to combine baseband and AP on one chip.

(x86 has a truly atrocious thing called SMM (“system management mode”). It is not directly exposed to anything except firmware and malware, so it could be changed in backwards-incompatible ways without breaking existing software, but for some reason neither Intel nor AMD seems willing to do so. For desktop and server workloads, this isn’t really a big deal. For something like an iPhone, it would be a major non-selling-point.

Also, x86 interrupt latency is awful, and this is an ISA issue. x86 cannot currently compete in any application where this matters. FRED may improve this if the implementation is good enough.)

You could even get an AMD chip in your XPPhone https://www.umpcportal.com/products/XPPhone/XPPhone :p

The biggest issue is that ARM was already king, so x86 was an exception that wasn't always accounted for.

So there was occasional incompatibilities and no real benefit to balance them.