Some young players, whose lives have revolved around BB for the last few years, can not handle the fact that they are not playing (even though they are practicing). They get themselves in trouble - inside or o/s the university. Other players use the redshirt year not only to continue to develop their game, their strength, etc. but they also use the year to adapt use to college life, going to classes (making the necessary grades in some cases), freedom from parents, being away from their GF, etc.
When a coach has a concern about the first possibility, they are much more apt to agree with an athlete's request not to redshirt. There have been numerous examples of this over the last 20 years on the men's side. In some cases, it was absolutely the right decision. In other cases, it was not.
I will also comment that I think the majority athletes know before they arrive on campus whether they will redshirt or not. That discussion is part of the recruiting process. Some kids are very agreeable to redshirting, others will rule you out if they think they will be redshirted their first year.
My comment above was not intended to relate to Gentry and Antwi. I have no insight into their situations.