At the first time I watched the scene I wondered the same thing.
Now I guess It's because he wanted first to let Simba believe he killed his own father, and he thinked that If Simba goes away from the PrideLands, he wouldn't have food or water and then he die, but his plan didn't work because he was saved by Timon and Pumbaa and brought into an oasis. Also he send the hyenas to kill him to be sure he would die, another probability is because he was lazy.