This has been discussed so much but I'll bite anyway...
1) People take longer trips to WDW. When my parents took me back in the 90s we visited the three parks only once. You spent one day at the MK, one day at Epcot, etc. If you had small children it would be much easier to "push" them to walk for a couple of days instead of for a full week (or longer). My kids were pretty much out of strollers around 3 years of age but still used them at WDW. Even with taking midday breaks, naps, and pool time... a week at WDW is a lot of walking for little legs.
2) Strollers are more compact and easier to take on vacation with you. Yes, I know you'll still see double wide strollers that are about as big as a compact car but for the most part a lot of the strollers are small travel strollers and umbrella strollers. Plus, even the big ones fold up quite small and are easier to travel with. Most people didn't have strollers back in the 90s that were compact enough that they wanted to travel with them.
3) Strollers have higher weight limits now, even the small compact strollers can hold children 50lbs. Children may have been forced out of strollers "back in the day" just because they couldn't fit anymore.
My kids were out of the strollers by age 5. I think most kids are ready to tackle the parks by age 5 BUT I don't like to judge other parents. Even at age 5 there were times when my kids were really dragging their feet. Sure you can take breaks and rest but there are times when they just have to walk (at closing time, etc.) One time my son got quite sick at the MK and his sister so sweetly gave up her stroller so her big brother could sit down. I'm sure there would be people on this forum that would have turned up their noses at us if they saw him in the stroller without even knowing the situation that day. I try not to judge when I see "bigger" kids in strollers. I have no idea what is going on with that chld and that family. I am quite content to take my children by the hand and go walking, if a stroller gets in a way I just smile and say excuse me. There's a lot of things that could ruin your vacation... other people's strollers ain't one of them!