The difficulty adjustment only goes up by a certain amount, and you'd only expect mining and difficulty to normalize when it's a margin driven business. If there is some limiting factor (e.g. not enough hardware, not enough power etc) or if price is increasing faster than bitcoin mines can come online then the difficulty adjustment can't keep up.

Other coins have solved this, in fact bitcoin cash had to solve it as soon as the fork happened because it would have taken too long to hit an adjustment period.