XBox 360 and PS3 (the latter in particular) are difficult to develop for, have all sorts of licensing restrictions, and are virtually useless unless you have a development PS3 or XBox 360 (those are hard to come by AFAIK).

The option is provided by other popular commercial engines -- going in that direction means GS is just playing "catch up". By supporting all major PC operating systems (which Unity doesn't), it's not just exposing us to a 15-20% larger market -- it's exposing us to markets where games are relatively sparse and gamers are plentiful and desperate(/generous).

EDIT: Ninja'd by Rondidon, but I think we both have slightly different points, so I'll leave mine as is