The TL;DR of why is that without ground clearing fires there is a build up of trees and debris, resulting in what would have previously been a patchwork of groves of trees turning into thick continuous forest coverage with no breaks. In addition to this the ground gets cluttered with fuel like dead trees, needles and the like, and there is a build up of intermediate trees that help carry any fire on the ground into the canopy of the larger trees.

Paul Hessburg of the Forest Service and University of Washington has a good TEDx talk on this that helps explain it: https://www.youtube.com/watch?v=edDZNkm8Mas