Because going out of town had many problems of it’s own that were only by those who went OOT themselves.
The parents thirty years hadn’t done so and were blissfully unaware of them
To give but two of the many (1)the difficulty of getting enough sleep in a dorm (2)having a vulnerable teenager in a situation of 24/7 peer pressure.
When I was in my early twenties in the late 90s I was at a large debate about OOT with about thirty bochurim present. The only ones who said they would send their children OOT were the ones who hadn’t gone OOT themselves. Some of those who I remember as the biggest talkers about how they would only go OOT thirty years ago are now some of the most vehement opponent of OOT