Well, MGS2 and 3 have existing engines that need to be ported, which would only require the devs to accomadate the platform transition. Peace Walker was probably more complicated, as it worked on the PSP. The likely reason their excluding MGS1 from the collection is probably because of the fact that the last remake was the Gamecube, which is owned by Nintendo, who, being Nintendo, would probably not give up the rights to Twin Snakes. Konami probably wouldn't bother recreating the first MGS, regardless of whether or not they use the existing engine of Metal Gear Solid 4 or otherwise.
Also, it's cross platform, meaning that you can get it on the 360.
Oh, but MGS1 was soo good...
360... Which I also don't have...