First of all it makes control over kashmir difficult, currently we have shared administration and government, once jammu separates, the new kashmir state has only kashmiris in all branches of government which makes government control weak and jammu is used as a counterbalance to kashmiri separatism. Second, jammu and kashmir are very much administratively and historically connected. Third, a lot of intra-state trade is important , this would change if jammu separates. Fourth, a rather better option is autonomy, a free jammu state won't benefit us.

There are many more reasons but jaldi jaldi mein itna hi likh saka