The reason Goguryeo failed to unify was simply a two-front war, nothing else.

If they could have projected their full military power solely toward the south during the reigns of Gwanggaeto the Great and King Jangsu, unification would have happened.
But during Gwanggaeto's reign, after sending 50,000 troops to repel the Japanese forces, they were stabbed in the back, and it took several years to reclaim what was lost.
During King Jangsu's reign, they pushed Silla as far as the vicinity of Pohang, but as signs of a two-front war reappeared, they couldn't push any further.
It was a two-front war,
and one of those fronts was an enemy nation many times larger than Goguryeo itself.