I know that Rudy can be charitably described as "a controversial figure," in this sub, but I think his theory on the bizarre handling of Iconic Masters is probably correct. We got the full spoiler months in advance, which did a lot to kill the hype. The thing is, we got these spoilers very shortly after the massive Ixalan leak.

Basically, WotC got wind of someone planning to leave the set, so they beat him to the punch by releasing the whole spoiler themselves first. The fumbling of Iconic Masters from that point forward is the result of an early, unintended spoiler and hype-killing consequences not only of the set's contents being publicly known, but also being unexciting compared to Modern Masters 3. The end result was that the hype machine for this set was crippled very early on, and there's only so much to be done after that. So WotC cut their losses and accepted that this set was going to be underhyped and it wasn't worth the cost of trying to punch it back up.

Masters 25 will be the proof on this if they handle it differently, assuming it doesn't also get leaked.