The design predates broad availability of jetways at airports. The concept, in the 1960s, was that by being able to use stairs they'd be able to service many smaller airports. By the 1970s this was clearly the wrong choice -- smaller airports just installed jetways too. This was a design decision that didn't even last the decade, but was brought forward for the next 50 years because changes would mean a recertification. That is why the landing gear is short. There are essentially no operators who make use of the low height of the aircraft, and there hasn't been for longer than most of us have been alive.