Also, by focusing on taking older females (which usually do have fawns), you are less likely to accidently kill a button buck.
There are typically several reasons why we're choosing to take a doe.
A common one is to improve the buck:doe ratio ---- mistakenly killing a button buck is counter-productive towards that.
Another reason is to increase the number of males that do not disperse from their birthplace areas.
Orphaned button buck fawns tend to remain in their birthplace areas, whereas non-orphaned buttons tend to disperse over a mile from their birth areas (which in many cases, removes them from the property on which they were born).
So, if you kill a doe that doesn't have a fawn, you have zero chance of preventing a button buck from dispersal.
Meanwhile older does tend to have more fawns.
There's also typically a little more tenderloin on an older doe vs. a younger one. :)