Because George Lucas is an IDIOT
Or you could try:
Because Force choking/crushing of organs is a Dark Side power.
Obi-Wan had no other weapons at his disposal. His lightsaber was dropped (recovered by Commander Cody), the electrostaff was useless. Only Grievous' blaster remained. Rather than use a direct Dark Side power, Obi-Wan simply called the blaster to his hand and used it as a last resort. After using the blaster, he disposed of it, stating: "So uncivilized." Sounds like that was pretty much in character with Obi-Wan to me.
Sorry, I don't have the plugin to see what your image/gif/video is.