Ted provided a good explanation for why the triathletes chose 650 wheel. It seems reasonable to me. However, it needs to be pointed out that the Trialthletes use 650c standard not 650B.
Sheldon Brown cited standards. 650c uses a 571mm tire bead diameter. 650B used a 584diamter tire bead diameter.
I learned of this difference the hard way. I acquired a set of machine made 650c wheel in hopes of getting a wheel "deal" for my ordered but not delivered 650B bike. The 650B bike on order would use cantilevered brakes. The bike eventually came and I was horrified to discover the difference as I attempted to mount the wheels to the bike. So the 650c were sold and appropriate 650B rims were acquired and wheels were built.
Ted's comment didn't indicate but it is my understanding that some wheel sizes exist to allow different width tires having the same tire tread circumference. Rivendell Reader and some web sites have discussion of the ease of conversion between 650B and 700c (except for bikes using non-adjustable cantilever brake calipers).
All this is rehash of old stuff so I'll stop for now.
Todd Teachout
Hercules, CA