I wish I could say the decision to arm the Stryker with a 105mm was the result of the sort of close and careful analysis many on this site like to do in their spare time, but the fact is that the Army had a bunch of old 105 parts and 105mm ammo in warehouses, depots and industry were still tooled up for that caliber, etc. It was pretty much a question of cost and convenience. Also, to remind everyone, the Stryker was supposed to be an interim vehicle, off-the-shelf, to get us through to the FCS.

As for using variants of M113, the Army leadership early on - we're talking 1999 here - decided any new vehicle would be wheeled, not tracked.