I think the difference being how long it takes an edition to get there. Look at 5e - IIRC, 4e had more books by year 2 than 5e has in 5. Quickly looking at the list on Wikipedia- I see 41 Player and DM books. Plus nine setting books. O.O In 4 years. So, yeah, we're looking at a hardcover pretty close to every month. That was insane. No one could ever keep up with that pace.
Even though arguably 4e is the shortest edition, in it's time, it managed to bang out almost as much material (not counting adventures) as 3e. Good grief. I had kinda forgotten how ludicrous it was back then.
You're right. Even though its shelf life was not so long, its product line, was. I think Heroes of the Feywild (the Witch is very cool) was their best latter day piece.