I believe that Disney does so because their dominant market share allows them to coast along adding new attractions only as fast as required to keep bookings at a healthy clip.
Whereas Uni is obliged to whip the horses so to speak in order to try to catch up.
To some extent this leaves Disney in danger of appearing arrogant and their parks looking stale ... hence at least half of the discussions around here ... but it's clear that they are 100% confident in the "timely" (meaning relatively slow) introduction of new attractions in order to maintain their lead.
It's the old rabbit / hare thing ...