The Key to Cage Match Victory: Easily Walk Out.

Why is exiting the door a win condition for steel cage matches in WWE?
I'm fine with pinfalls and submits granting the win, because they still
fit in with the spirit of a caged duel straight out of Bloodsport or
something. But this door nonsense, it's no good. Cage matches are
supposed to be brutal, drawn out affairs. A big guy's going to have an
easy time knocking out his opponent, but climb really slow. A
cruiserweight's the opposite: he could sprint up there, but it's going
to take forever for him to knock out the other guy.
But once the door was made a standard win condition (sometime around
HHH's reign?), it felt like matches should last ten seconds. Just signal
the ref to open the door and suicide dive through the ropes.
So why, Scott? Why the door? Why?!
– Zeus

The door was a standard win condition in WWE cages matches WAY before HHH was around.  The original "escape the cage" stipulations came around in the early 70s so that Bruno could beat guys like Stan Hansen without them having to do a "real" job.  It just kind of became a thing for a long time, and then in the late 90s the company got an influx of all these guys like HHH who grew up with the NWA on TV, so it changed to pinfall rules for a while with the advent of Hell in the Cell.  And now it's a bizarre hybrid of the two, a seemingly contradictory set of rules.  I have personally always HATED escape rules.