Because ones hands are easy to remember and recognize how they suppose to look like. To achieve desired result you must build a habbit to look at your hands every once in a while so you'll do it as often as once in 15-30 minutes for at least a week. And looking at own hands from time to time in real life is one of least weird things you can do. So it's a perfect trigger to build habbit around.

And how it's actually work is already mentioned in other comment - when you look at your hands while dreaming they will look different or weird every single time. This is suppose to make your consciousness go into "wow I am in a dream" mode.

It's like ChatGPT: hands are difficult to hallucinate and don't stay consistent throughout multiple viewings