GM knows their audience and what names have more presence or awareness. Naming it Silverado means it's familiar and more eyes will look at it.
Because the name Avalanche is dead. Avalance never was very successful, anyway, so there's no reason for GM to bring it back. Silverado has the recognition they need and want for their EV.
............................but then EVERY PERSON who sees the truck immediately thinks Avalanche. So what did they accomplish?