I don't have any inside knowledge of how blackbox works the way it does, but I imagine, like with any critical system, reliability and redundancy trump everything else. I bet the two hour limit is not an arbitrary number but rather result of some lengthy tried and true testing process and debate. Any design decision is a trade-off, no matter how small.
I'd argue that, if we have learned anything from MH370, ensuring blackbox survive in the harshest environments in the planet and stay discoverable for long period of time should be the utmost concerns.
