Why didn't Light just kill the Task Force? I know that Light didn't want to kill his father in cold blood, but after his father, Soichiro Yagami, died, he could've just killed them. And then, Near accused L #2 of being Kira.

Near only found out that Light was Kira because of the information Aizawa gave him. He could've just killed the Task Force and went incognito. I know he wanted to kill of Near and Mello, but I'm sure that he could've found a way to kill them off later. 

Why didn't Light just kill the Task Force then?
To add to Vixen's answer, Light didn't kill the Task Force before Near was even in the picture because they posed him no threat. Initially, he only killed criminals, and later he started killing everyone opposing his utopia. However, they were working for him then, so there was no need to kill them.

Also, keep in mind that he couldn't hide those deaths from the world. It would seem rather strange that the entire Task Force died and he was the only survivor. He would have to made them all die in such a way that he would seem innocent, but it would still seem odd to any onlooker. So, maybe he didn't want to risk exposure even more.

On a side note, it might have been his conscience if he still had any left. He partly killed his own father to convince everyone that he was innocent. He purposely made his father do the eye-trade so that his father could prove his innocence by telling everyone he had a showing lifespan, meaning he could not have been the Killer. Killing everyone else after that ordeal would make his father lose his lifespan in vain.