why is variable difficulty not allowed in runs?

Do you mean automatic difficulty adjustments? If that is what you mean, it's fairly simple:

If automatic difficulty (AD) was allowed, it would be much harder to accurately judge a submission's difficulty. Most people would have a different opinion about what category a submission would fit into. This could lead to issues with multiple runs being misjudged and put into the wrong category.

If the community is in favor, an automatic difficulty category can be created.

i asked this mostly as "why doesn't that exist" honestly but i could have made that more clear