Because it is newer, faster, has DX12, sleeker design, etc etc etc.
If I'm not mistaken, upgrading also nets you a legit Windows 10 license, regardless of whether or not you upgrade from legit or pirated.
Also, Mahjong + Solitaire collection, fuck.
Why free? As i see it, MS wants as many people as possible to switch to 10, to become users of Microsoft Store, buying apps from that. That thing never really took off with Win8, after all, yet is still a mostly untapped and possibly highly lucrative business opportunity.