Because it will make balancing encounters and the adventuring day easier. Short rests would make both much easier. Long rest recovery opens up the 5 minute adventuring day problem, but I'll admit that's more of an issue with planning than it is at play (I've only seen it happen a few times, groups tend to be pretty good about spreading their abilities out).My question is why are you doing this?
Long rest recovery would lend itself to more of a D&D feel, with resource management as part of the game. Short rest recovery could lead to the 4E 'tactical miniatures game' feel, which I wasn't opposed to but I recognize it does strain some people's believability. Long rest recovery does have a more "realistic" feel to it too.