Because Rommel wasn't that good. Rommel was good as a traditional cavalryman. Lots of dash and capable of the quick win. What he lacked was a strategic plan to hold what he won. If you look at the North Africa campaign Rommel started off spectacularly pushing the British back to Tobruk. There he found a fortified nut he could not crack.
The British regrouped and pushed him, in turn, back to his original positions.
Rommel then got reinforced and made a second push much like the first against a weaker British force (they had withdrawn troops for the Far East this time, Greece the first time). This time Rommel got as far as Alamein where he again ran into a fortified position too tough for his little force to crack.
This time the British reinforced greatly and later assisted by the US pushed him in the end completely off the African continent. This does not speak well of Rommel's strategic vision. He obviously was a good tactician and could virtually run circles around most of his opponets on a battlefield where things were fluid. However, he lacked the resources and ability to deal with a dense battlefield and attrition warfare.