The same reasons I frown on all hybrids.
1. All it takes is one dippy politician getting their hands on info about hybridization thinking that means ANY ball python could be a Burmese python and thus dangerous under whatever BS law they have in that district. They see that and ANY species could be a hybrid rattlesnake!
2. Diamond Pythons and Sinaloan Milk snakes are two species practically lost to the hobby because of hybridization that I can think of off-hand. One is so ridiculously expensive that it is basically lost, the other is indistinguishable from nelson's milksnakes and only "shows" as a Sinaloan because of line breeding efforts with no one able to trace lineage back to the true species.
3. Using hybridization to get ball python morph genetics to another species is a road to people lying about what they have. You use the example of getting a coral glow Burmese python. But you just lied right there, because it isn't a Burmese python anymore. It's just some hybrid. You cheated essentially. Then when someone finds some great genetics with his pure Burms no one will believe him because they "probably just crossed it with some ball python somewhere down the line."
4. Hybrids don't have nearly the value people think they do. One only needs to look at the cornduran or jungle corns and how long they hang on the classifieds even though they are very cheap. Even pure plain Jane corns sell faster.
These are just four reasons. I have other reasons based in my desire to keep lines clean because if I buy something I want it to be that specific species because of the natural history aspect of it.