Expansion is right around the corner, IMO, and the Suns sale is probably speeding it up. Adam Silver gets to see who all has the cash and interest to buy a major franchise. Only one person, or group, will purchase the Suns though and that will leave a half dozen suitors who will have to wait for the next sale. Of course not all will want to pay as much for an expansion team but some of them will, which is key. Silver can create the opportunity for them.
Both Seattle and Vegas have NBA ready arenas right now, which means they'll be good for the next 4-5 years but to wait longer might age them too much and require substantial costly upgrades so there's another cost in waiting too long.