Because the queue system is completely broken and people are getting stuck on the load screen until they timeout.
https://us.forums.blizzard.com/en/wow/t/dungeon-queues-broken-on-oceanic-servers/748853/2
https://us.forums.blizzard.com/en/wow/t/cant-enter-the-dungeon/744797/36
https://us.forums.blizzard.com/en/wow/t/dungeonsrbgs-crashing-on-load/748830/4
https://us.forums.blizzard.com/en/wow/t/dungeonsrbgs-crashing-on-load/747151/6
https://us.forums.blizzard.com/en/wow/t/i-cannot-enter-dungeons/748772/7