As the older 3G networks start to bog down from age and the weight of usage, it only makes sense that the carriers would want to offload most traffic onto the newer 4G networks which can handle more traffic.

I'd say part of the problem for T-Mobile is that their 4G HSPA+ network is pretty much the same thing as their 3G network, so they don't have the same 'out' until they start deploying LTE.