Because they don't understand it mostly.
Almost everyone who has done the study or training for SMS can see how it helps. The only ones I ever hear complain are the ones that don't understand.
I include myself in that, I thought it was bollocks and complicated too until I went out of my way to get a full understanding. Now I got the jist and its easy and effective to use.
Yep it takes a few days to implement but after that probably less than an hour a month and it does improve safety.