Why does Arduino use Atmel versions only

Arduino is irrelevant or you'd be asking why is Arduino so popular and not why AVR. AVR's have been "popular" for a while completely regardless of Arduino. Arduino is a product of the same things that made AVR attractive. It's another product, another devkit. AVR is not popular in units delivered where 8051's in devices and PIC's in smartcards or whatnot have massive numbers or in cellphone and PC markets where the AVR doesn't even compete.

Instead of popular, you could say AVR is attractive. And it is. The main points already came out: It's available and inexpensive, requires very little components or board features (clocks, buses...), is easily ISP and above all, there's good software support. You get a real compiler, programming software and hardware designs, docs, samples, libraries, all free as in freedom. You don't have to fight vendors and wonder if you're code size limited or hear that your compiler has been discontinued or won't run on any operating system from this decade. If not before, then once you've been bitten enough times, you'll appreciate open tools and docs where nobody dictates what you can and can't do or know.