Same argument as questing zone layers , to divide the pressure on the zone so it won’t crash for everyone. This is the short version.
Spinderella:
500 ppl in Orgrimmar without crash ,but on sod thats limited to 10 ppl per city cause 3 thousand layers ?
Yes. There is no need to insult me in the short version, I have quoted the exact part that applies here for your question in the longer version since you just dismissed it without reading through:
This means that at peak times, there may be 5, 6, 7 or even 10 (or more) layers going at once on the largest realms. At off-peak times however there would still be that same high number of layers and several days after weekly restarts, there would be so many layers going that late at night or early morning the world would feel very empty. This means even if several thousand people—what would normally be a “large” realm in earlier versions of WoW—are logged in at 8:00 AM but there are 10 layers going from the previous day’s peak, each layer only has several hundred people (when it could handle several thousand) and it could feel pretty lonely.
For Season of Discovery, we decided to try something new and allow layers to automatically retire when populations dropped to “collapse” the number of total layers and make the world feel more active and lively at all hours of the day and night. We hoped to avoid those “dead” periods where the realm felt more empty than it actually was.
The unfortunate side effect of this is that every time we would retire a layer, it would suddenly cause players to change layers as the layer manager re-balanced and redistributed folks. We’ve never engaged this system before because we knew this would feel jarring, and in a game mode like Hardcore for example, being unexpectedly moved to a new layer when yours is retired could be deadly! But, like many other aspects of Season of Discovery, this was a risk we consciously took to see if we could improve the experience for those that prefer to play at off-peak times.