Because DL is a local’s park, people bring too much crap in. Lately the trend has been the Mega Strollers and wagons (which are allowed only for special need children but people know they can just say that). The machines used at WDW aren’t effective in screening tons of crap and will set off an alarm anyway.* So until something better is designed, we’re stuck with this set-up.
*there’s also a greater concern at DLR in people bringing weapons into the park
