Because 9.6v is a nimh battery which doesn't deliver the power as efficiently maximum of about 12c I think.You get slighty better results out of a 7.4v 25c+ lipo battery than you do out of a 9.6v nimh. The lipo is smaller, more power efficient,holds charge better and will last longer.
Nimh batterys can work fine in a really well set up gun (I use them with one of my gun set ups) but rof and response in a standard gun will be better with lipo as its discharge rate is higher and that makes it more instant on the power.
There is a lot more to this explanation,
Google lipo vs nimh and have a read as I hate typing out long posts.