The reason why is fairly simple if you think about it for a minute.
Israel has seen the reaction to Spain's withdrawal of support in Iraq and this comes right after the terrist bombings there and half the world (possibly even the terrorists) are claiming Spain withdrew because of terrorist pressure.
Now put yourself in Sharon's shoes. Israel is about to vacate the Gaza Strip. The message Israel wants to leave firmly in the mind of the PLO and others is that Israel is withdrawing on Israeli terms and no one else's pressures have anything to do with the decision. This way Hamas cannot claim a "victory" when Israel withdraws. Furthermore, killing Yassin is also a "freebie" now. Once Israel withdraws this will quickly be forgotten and they are up against one less capable motivator/planner in Hamas.
But then what does an Ass know?