Fiber is the answer
The carriers have/had to upgrade their back-haul (the connection from the tower to the Internet) in order to support 3G. Historically these connections have been 1.5Mbps T1s in the US (slightly faster in the EU with E1s). They only have limited options:
1) they can go with multiple T1s, but it takes 5 of these to support 7.2Mbps
2) they can go with a T3 (45Mbps), but these are expensive
3) or they can go with fiber (up to 100,000Mbps and growing). This is slightly more expensive than a T3, but has the advantage that it *NEVER* has to be replaced again - just the electronics on either end.
Naturally, the carriers are choosing fiber as the correct upgrade path - and since they are paying to have huge amounts of back-haul bandwidth available, they might as well SELL it! The limiting factor is simply the electronics in the handsets, hence the testing.
Unlike traditional carriers, Clearwire started with fiber, and is anxious to be able to sell that bandwidth to the most people by whatever technology is available and popular. Currently WiMax is the only game in town, with LTE coming in the near future.