The laws concerning the red heifer don't make a lot of sense to us, and God didn't put a lot of ink in Scripture to explain it. He told us what to do, but not why. I can think of four reasons for this off-hand:
- The spiritual mechanism behind the red heifer might be beyond our comprehension. Explaining them might be impossible or just confusing.
- Some knowledge might be deliberately withheld from mankind because it's too dangerous for us to have. The Torah concept of witchcraft appears to be related to accessing or using forbidden knowledge and the hows and whys of the red heifer might fall into this category.
- God just wants us to trust him on some things, to obey without knowing why. It's a test.
- It could send us off on irrelevant bunny trails. Leaving distracting information out keeps us focused on what's important.