Odd indeed that the pax were not flown down.
Diversions are usually fast moving and dynamic events, with events often conspiring to make you look like an idiot/liar - the Law of Sod ineviatably kicks in.
There's a fair chance that they were sat there with a forecast improvement, but with no change in the actuals, then no change, then no change and then as soon as you get the pax off the a/c, weather starts improving....