The direct reason is that, after decoupling, HMaster does not extends HRegionServer, so we do not have RegionServerAddressTracker now, so for backup masters we do not have a way to get all the region server which means it can not serve as connection registry any more, so we need to find a way to let backup masters to also have a way to get the region server list.
But in general, since we already have a class in HMaster which is used to track region servers, we'd better reuse it. And after this change, the backup masters could also be used as the fallback servers for getting region server list in
HBASE-26220, which could also reduce the load of active master.
And this could also speed up the active master start process as we could save one zookeeper scan when becoming active master.