Turn around time is a major factor for airlines, particularly low cost ones and you’d be surprised how long it takes to get 200+ passengers off an aircraft. Only then can the security sweep, cleaning and restocking begin. Single aisle aircraft can be worse as it only takes one pax blocking the aisle for the whole process to grind to a halt.
It’s not always possible to use the rear door as the gate may not be equipped with two aero bridges and if steps are used there has to be security in attendance to monitor the pax because they can’t be unsupervised on the tarmac.
Built in aircraft steps aren’t as good as they seem, they add to the purchase price of the aircraft, are dead weight flown around and require maintenance when they break down. Typically pax can only embark and disembark in single file and there may be a limit to how many in total can be on the steps at once. Ground based steps are much stronger, allow dual file and are not normally limited in how many people can be on them at the same time.
Every minute counts, with turn around times of around 30 minutes being targeted.