Lots of reasons why people aren't keen.
These are necessarily my own views but ones I've heard of in Real life or on Mumsnet:
(A) cost, time, inconvenience and/or practicality of finding childcare especially if overnight; not everyone has free local childcare at hand; not all children can be left (baby, breastfeeding, disabilities or special needs, can't be left overnight, etc)
(B) children not seen as part of the family, especially for a family wedding; we wouldn't think it's okay to exclude other age groups so why are children fine to exclude
(C) bride and groom get cross or annoyed that someone can't come to the wedding due to children not coming; b&g should always accept that people can say no to an invitation, especially if their reason is a consequence of the b&g's choices
(D) limited family time off work so don't want to spend some of it without their children esp of the wedding includes a fair bit of cost, travel time and/or holiday days from work
(E) when the b&g say the reason is so that the parents can enjoy themselves when the actual reason is the b&g don't want children there for whatever reason
(F) a dislike of children only being allowed to go if they are wedding props for photographs
I have only been (invited) to two child free weddings despite going to many many weddings over my 40 odd years. Been to dozens over the years but pretty much all have included child guests.