Because it's logistically im-****ing-possible. The Isrealis want no part of it, and neither does the Iraqi government, or any other neighboring country, because in all cases they quite rightly look at us basing operations into Syria out of their territory as something that will create a whole lot more very bad problems for them than it could possibly fix.
Syria is not a thin crust of cities like pearls on a chain along the shore like Libya, it's throroughly settled with many cities all over it, a large land territory and a large population, a whole lot more like Iran. No matter what Obama and Clinton might "Feeeeeel" like doing, this is one where the Pentagon would say 'Aw, Hell no!'