Actually Sprint and T-Mobile have never officially tried to merge. So far all the executives have done is deny that such talks are in progress.
At one point AT&T tried to buy T-Mobile, citing Verizon being bigger as why it was okay. They even put a bunch of money on the line if it fell through. Regulators decided that either they wanted 4 parties, or that Verizon would be forced to buy Sprint to compete, leaving 2 parties. (I am not 100% certain which they said was the reason)