Don't forget that Muslims had conquered a lot of area that had previously been controlled by the Romans, such as Egypt, North Africa, and Iberia (Spain and Portugal). And much of the Muslim world retained cultural and trade contacts with Al-Andalus, or Muslim Iberia. And there remained contact with the rest of Europe and Christian pockets in Spain under Muslim rule. Plus, their relationship with the Eastern Roman empire was consistently antagonistic (they were the ones who invited the Europeans there) but not always exclusively so. By that I mean that there was trade contact between many Islamic state and Constantinople. And after the Caliphate began to be broken up Constantinople even had some diplomatic contact with some Muslim countries.

Long story short, there probably wasn't much the Muslim world needed to learn from the crusaders. Because they never really lost contact with the West. Some of the works "rediscovered" by the West through Islamic contact were ancient Greek works. Islamic scholars had been particularly keen on preserving and expanding upon Greek works of math, science and medicine/anatomy.

I'm not saying that they learned nothing, at all. It's possible. Especially when it comes to siege craft or fortification. I don't know. But the reason the focus is so much the other way is because of the continuing and extensive contact the Muslim world already had with the West.

(And, yes, much of the West had contact with the Islam world, as well. Especially Italy and the non-Muslim parts of Spain. But also in the far North. Which is why some have made the case that this learning stemming from the crusades is exaggerated. There are certainly stories of clergymen going to Al-Andalus to read in their libraries. The world of the Middle Ages wasn't as closed off as people think. But it is also true that for some of Northern Europe this would have been a first exposure.)