It's because it wasn't (and still isn't) a reservation in the standard sense of the word. You may have an ADR or had a PS for 10:30 at Crystal Palace, but you might not have been seated until 11 because no one was leaving the restaurant because it was raining so hard. It's more of a hold your place in line thing.
What do lawyers have to do with using the proper words for what you’re getting?
I have never once in my life had a “reservation” where they’ve managed a restaurant so well that the table is available exactly at the right time. Yiy pretty much have to keep a ton of tables empty to do this. I don’t eat at auch fancy places.
All I’ve ever gotten is priority seating in front of walkins, but that’s still up to available tables. If I’m a group of 2 and there are groups of three and four in front of me, I don’t expect to get a 4 top ahead of them IF there are 2 top tables in that restaurant.
PS is a much better term for what you’re getting, even in the era of ADR.