I think it was a mix of things. He dropped his 1st two albums in the same calendar year. And then he dropped his third album maybe 1 year, tops, after that.
3 albums in two years back in those days was a LOT of material. A lot of it started sounding very similar too. The singles started sounding the same. There was more and more Swizz Beats and less variety on production. He started running a few concepts into the ground, like the Demian series and the Prayer/Gold conversation final tracks.
I think to some extent he over-saturated the market during a time where the tides were starting to shift anyways. Then Dynasty/Blueprint, Rocafella, and The Diplomats kind of started changing the sound of the culture.
Usually when someone comes in the game and takes over with really hardcore raps, it works for a short period of time. 50 was kind of the same way. Everyone used to say 50 DMX'd the game by knocking out the flossy raps with more aggressive content. That works, but people also get tired of it kind of quickly.
And finally, DMX's 1st album is a true classic, and DMX is a dope rapper, but he's not elite enough or versatile enough as an emcee to drop multiple times per year without wearing people out.