Are you asking the reason they do it or the reason it's controversial?
If you're asking why it's seen as controversial, that would probably be because people suspect a much older partner with a younger one might be taking advantage of the younger one. This is a more recent attitude. Historically it wasn't that rare for larger age differences in marriage, especially of older men to younger women, because society valued men for the capital and status they may have and women for their reproductive capacity; an older man was likely to have accrued more wealth or respect over time than a younger man, and a younger woman was probably seen as being more fertile/fecund than an older one. Biological roles influencing culture, basically.
But I notice the controversy about age goes both ways now. You can be an older woman and people will think if you're dating a younger man you're messing around and not serious about it or he is a "boy toy". If you're an older man dating a younger woman, people will also think you're not serious about it, or are creepy, or taking advantage. My personal experience of having a serious partner for 10 yrs who happened to be 6 years younger than me is that yeah, people think it's mildly controversial. They assumed I was taking advantage, which I would never do. While I don't think this should be enough to put someone off love, it sure is annoying to deal with it from friends, family or complete strangers alike. Maybe it's enough to put some women off doing it.
I usually see women I know going for guys who are the same age or up to about 4-6 years older than they are if they are looking to settle. There are a few exceptions, but I guess it may have to do with them preferring men who have stable jobs and are less flaky. If they are just looking for some fun, I've seen them care far less about the age of the guy.