They don't have enough capacity with their popular attractions.
To beat the dead horse yet again, this is their fault for failing to invest in expansion over what is now approaching the last 20 years while continuing to market heavily and push to increase year-round attendance.
It was a problem with FP+ that you could do the same things over and over when they may have had enough TOTAL capacity* but not enough capacity for the things people wanted to do.
I'm sure this part of the change with G+ is their attempt at "correcting" since they're almost impossibly behind now and have apparently lost their institutional knowledge on how to build appealing high-capacity attractions.
Yeah, it's galling that they put this guest limit on the new service they charge for when they didn't on the free service it replaces but isn't that pretty much in line with modern Disney's managements idea of the Disney Differenceâ„¢ - pay more and get less?
*COP, I love you but most people don't pay $125 a day to sit in a musty smelling theater and watch a show with failing animatronics and a horribly outdated ending that needs to be retooled to represent the end of the 20th century with an optimistic look forward rather than pretending to be the near-term future it isn't. Don't act like this attraction that's gotten no love in 20 years (like CBJ) going 4/5 empty all the time somehow offsets the frequent modern waits for just about everything else in the park including other attractions just as old which have gotten real refurbishments and updates.