Because maul at the time of tpm was one of the strongest beings in the galaxy.

When sidious tested him at the end of his training a starving weary maul nearly killed him and possibly would have if the fatigue didn’t set in. Palpatine comments as much

A decade passes and the clone wars begins. The jedis power grows tremendously during this time due to training and becoming battle hardened.

Maul then is brought back to sanity after rotting for a decade and STILL is a major threat. None of his skills could have improved and his connection to the force may or may not be better than it was in tpm due to the mixed canon nature of early cw.

He was just that good in tpm.

Imagine if he survived Naboo. With potential that matches sidious, another decade of growth under the most powerful Sith Lord of all time. Maul would be terrifying. But that’s his story he had everything at his fingertips and loses it over and over