I would say that it’s because Flash on phone is pretty dead as it is, I’ve never missed myself on my 8X really. There’s HMTL5 today, you have Ajax and Jquery and. It’s not like Flash is going strong and microsoft/adobe wanted to exclude it from wp8.
And, it’s not only wp8. It’s getting killed off more and now. Nexus 4 doesn’t have flash support as well, which I think is a good thingh: just kill it already.
But, if you DO want flash: the New HTC One got that. Perhaps doesn’t agree that flash on phone is needed today, but apparently a few sites still uses flash.