Socially speaking, we tend to think of great generals as great tacticians on a battlefield, born out of desperate necessity.
Hannibal and Alexander were known for their creative positioning and planning by which they were able to defeat armies that far exceeded their own in size. Cannae being the biggest one for Hannibal, and Gaugamela for Alexander.
Charlemagne on the other hand, was not known for battlefield creativity. He had no need for it. He was the leader of an unstoppable juggernaut of an army (barring Roncesvalle) that spent as much of its time besieging as it did skirmish raiding, and thus comparatively less often in open pitched battle, except when he had at the very least numerical equality, but more likely superiority.
This is partly why one is hard pressed to remember Charlemagne's battles outside of Roncesvalle (which was more of an ambush of a withdrawing rear column) and maybe the siege of Pavia, which was more punctuated by the speed with which he reached the city and determination of his forces to remain at the siege site than any heroic slaughter.
His greatest victory was the capture of the Avar ring, which entailed no conflict. Likely because his forces were so overwhelming that the Avars didn't dare confront him openly.
This of course, has no bearing on him as a strategic thinker. He definitely would be closer to a Ulysses S. Grant than a Robert E. Lee. He wasn't a tactical battlefield genius, he was a strategic planner. He had a job to do, he got the army and the resources needed to do it.
Battles be damned, there's a war to win.
Sources:
What is the context of your question? Whose standards are you referring to? Who has called the latter two great generals but not the former?
Have you considered the importance of Charlemagne's role as Holy Roman Emperor & political figure vs. the more heavily militaristic / conqueror roles of Hannibal and Alexander?