So Intel just hasn't bothered to getting around to designing a low-power x86 chip that can compete in mobile devices with ARM? The more likely explanation is that it can't be done because ARM is a better designed and therefore intrinsically more power efficient ISA than x86.
In any case, the more pressing problem for Intel is that in the mobile space consumers don't know or care what types of chips are in their phones so even if Intel could design an x86 chip competitive with ARM, consumers are unwilling to pay a brand premium for Intel that they're used to getting in the desktop market.
Intel recently released its "Medfield" SoC. It beats ARM cores at similar power envelopes, and is currently available in a few phones.
Of course, it's a single-core processor as opposed to the double or quad core ARM processors that it's up against, but the point is that they're getting quite close.
In any case, the more pressing problem for Intel is that in the mobile space consumers don't know or care what types of chips are in their phones so even if Intel could design an x86 chip competitive with ARM, consumers are unwilling to pay a brand premium for Intel that they're used to getting in the desktop market.