I think the big difference is software and software support - Deck has a dedicated LinuxOS and game developers explicitly tweak and target games for it. It has great configurability support for controllers, good store ratings for compatibility and a big community that automatically provides controller configurations for the games.
ASUS runs normal Windows which doesn't seem to behave all that well on such a small screened device so that's going to be a massive compromise.
I think everyone simply ignore or forget how performance-demanded the windows is. Using win 10 on hdd brings the hdd perf on system monitor to 100% half of the time, while it's normally operating on Linux. No wonder deck is simply more power-saving and optimized.
ASUS runs normal Windows which doesn't seem to behave all that well on such a small screened device so that's going to be a massive compromise.