First of all, the Crusaders are in the desert, in Cairo Egypt, and as we know, Egypt is a desert. A hot desert.
The temperature must've been high, causing at least Heat Edema to occur.
Just a few minutes before, all of the Crusaders (except for Joseph, who wasn't going insane, and Iggy, who wasn't even there at that time) were going insane, and they were still recovering, including Jotaro.
So Jotaro was in a weakened state, causing him to not probably be able to move at the speed of light, and Jotaro moves at speeds of about 833.7 meters when he caught the bullet (figured out by Darthplokoon2, thanks bud.) which was probably decreased due to Jotaro recovering from the heat.
Short Answer: Plot
Long Answer: Recovering from a possible Heat Stroke
How fast does Star Platinum punch?
(^^ Link to the question where I got the speed of SP.)