Mostly because it looks cooler; you can actually go to 3.00 front/3.50 back, but they look overly massive and the sidestand is almost too short. The bike leans so far it looks like it might fall over. The 2.75/3.00 sets look good. Check around or the Heidenau's, I bought 2.50 & 2.75's for approx $75/each from Cycle Outfitters (local Indianapolis area shops). Incidentally, although those are original sizes for the 175DT, they look really tiny. Do a search on this site, there are more than one thread on 175 and 350 tires that may give you some ideas.