They didn't.
It got folded into Pokémon Masters, probably as Sugimori intended, as it was his pet project and was being developed at the same time as USUM.
Giovanni arriving, in his Rainbow Rocket uniform, was one of the first major story arcs they set up, and has been a recurring narrative focus ever since. Even as recently as the most recent update, the dude has popped back up with a Guzzlord.
Pokémon Masters essentially continues what was set up in USUM, with Giovanni attempting to ally with (or defeat) the other villainous factions across the multiverse, and take over the island and dimension.
The game even implies some of the other villains are the very same ones in USUM too. For example, Cynthia mentions that she doesn't believe the Cyrus on Pasio is the one she remembers leaving behind in the Distortion World, and Cyrus corroborates this by saying he fell through an Ultra Wormhole (not a Hoopa ring as most trainers come to Pasio do) at Spear Pillar after successfully taking control of Dialga/Palkia.
The main games were never going to continue the multiverse story because it honestly is kind of lame and limits the franchise moving forward. Bringing it all into a mobile game so trainers and villains at various points in the timeline can butt heads is exactly where that idea belongs, and as I said, I suspect it was always the plan.
>TL;DR;
USUM's Rainbow Rocket is just a backdoor pilot for Pokémon Masters being the "multiverse" game with Giovanni and all the trainers new and old, in their prime, able to meet, talk, and battle.
