According to the arch wiki:
Why would I want to use Arch?
Because Arch is the best.
And let's not forget to consider this:
Why would I not want to use Arch?
You may not want to use Arch, if:
you do not have the ability/time/desire for a 'do-it-yourself' GNU/Linux distribution.
you require support for an architecture other than x86_64.
you take a strong stance on using a distribution which only provides free software as defined by GNU.
you believe an operating system should configure itself, run out of the box, and include a complete default set of software and desktop environment on the installation media.
you do not want a rolling release GNU/Linux distribution.
you are happy with your current OS.