As with any negotiations, the opposing party must have something to offer in return. In negotiations with Hamas, Hamas has something it can give in return - Gilad Shalit and Israel's security. Neither of these can be offered by Fatah. It cannot guarantee security of Israel because it does not represent the militant factions within the Palestinian society and it does not hold hostage an Israeli soldier. However, this answer is not complete without noting the political pressure on NT govnt to bring back Shalit and the timing of the Hamas agreement to the deal to coincide with the failure of Fatah to get UN backing for Palestinian state. Middle East politics is a complex issue.
US rally shows support for ex-Marine held by Iran on spying charges (AP)
from the article: NY Times: Shalit deal shows Netanyahu can reach Mideast peace, but won't