I'm guessing because people want to profit from mining the new coin and if the difficulty started out in the billions, that would be difficult.
Still though it's all relative. Some coins do merge mine from day one, like HunterCoin. They have a purpose for the coin (the HunterCoin game in the wallet) and so just need to keep transactions moving/etc, and instead of competing with BTC they merge mine. Makes a lot of sense.
I think any sha256 coin with some sort of real purpose to exist should merge mine. Only makes sense.