My first thought is that x86 is crappy, and that we only use it on computers for backward compatibility. But this is merely opinion-based. So, let just remember that most computers use x86 architecture, and that it would be expensive to switch to another architecture.
So, why don't Qualcomm use x86 on phones ? One part of the answer is that x86 is Intel property, and there is, to the best of my knowledge, only one firm that own a license for x86: AMD, who got it as IBM requested a second source for its CPU, as far as I remember.
And there is an other reason to maintain several architectures: to have architectures that fit the need. A modern x86 architecture requires much silicon, and much power. If you look at ARM architecture, you've got the high end ARMv8a, with much power and high power consumption (several watts), and, at the same time, ARM still develops ARMv6-M, with much less silicon (0.0066 mm2 instead of 9 mm2), reduced power consumption (200µW), but reduced performances (divided by 10000, roughly).
To sum up, we keep several architectures for backward compatibility, intellectual property, and purpose fit. You should probably study several architectures to get a better overview of this field of study.