Even when there are no legal limitations, sperm banks will often limit the number of donations made by any single donor. Most donors will not travel to donate to multiple sperm banks.
2) Limitations on donor anonymity. (probably useless these days - acquaintance of mine tracked down her anonymous donor father via 23AndMe, and was devastated to discover he didn't want to have any contact with her. seems obvious, but...)
3) Limitations on donor compensation. (same reason there are limitations on blood and organ sales, I imagine). However, you have to pay highly achieving men significantly more than others for them to take time out of their day to donate into a vial. And also to abstain so that their donations are high quality.
