A lot of it has to do with balance and minimizing any sort of nerfs to do it. Each level change affects the way your stats scale. +1 of any useful stat can be significant to a lv1 class, whereas +1 of any useful stat is fairly insignificant of a change to a lv50 (for the most part). Every occasion that levels get increased, it essentially means the devs are given a reset as to how stat scaling can work in relation to where the gear stats are at. It can also be a time to readjust the way certain stats or mechanics work. There's also the choice by the devs to make players experience new zones or content through the introduction of gaining more levels. Level cap increases typically signify some sort of significant change or hallmark that will revitalize the game, be it a lot of changes at once or one huge change, which is why it usually occurs with expansions. Be it new areas, new spells/abilities, new classes, new play mechanics, etc.
