There are many reasons for not adding a new feature. Here are a few:
1) Most people won't use it.
2) It makes the interface more complicated
3) People could change the max freq setting mistaking it for the min freq setting, resulting in confusion
4) The new feature would not speed up the process (due to the algorithm used)
5) The setting makes little sense from a linguistic perspective
So, it really comes down to a cost/benefit analysis. Let's see if anybody else thinks this is a good feature to add. I'm still skeptical and think the results could be fairly easily achieved by simply exporting the results to Excel and filtering the results there.
As I mention above, there seems to be no linguistically meaningful reason to limit the maximum frequency (at least to me). Also, Zipf's Law indicates that the number of n-grams above your arbitrary threshold of 5 are very few in number, so you're basically just ignoring a few very salient (and important) n-grams from your search. I come back to the question of why you would want to do this?