Originally Posted by
Thage
More importantly, why would the blood elves want anything to do with Kael'thas? The last time any of them heard from him, he led an attack on Silvermoon and took M'uru, which was a significant hit to their morale. Before that, he went south to help the humans, then fucked off to Outland for what, about a decade? And sent an advisor back to show the blood elves how to siphon mana as a stopgap measure in dealing with their mana addiction. The next they heard from him after his return, he tried to use the corrupted Sunwell to bring one of the lords of the Burning Legion to Azeroth.
Meanwhile, Lor'themar (though bland) helped pull them together as a people, sometimes taking drastic steps in the name of keeping things running smoothly in about as much of a worst-case scenario as a culture can have on Azeroth. Under his regency, the blood elves went from on the brink of total collapse, struggling to hold on to what territory they had, to their territory being largely secured, alliances providing them with backup in securing the Ghostlands, and they've bounced back enough that they can spare people for pursuits like establishing a rival archaeological society to the Explorer's League. In In the Shadow of the Sun it's even outright stated that Halduron and Rommath have been trying to push Lor'themar to establish a new dynasty (although, backed up by his NPC chatter in BFA, Lor'themar seems to believe maintaining the regency is sufficient and has no desire to see a new dynasty established, likely out of respect for King Anasterian who was the last King of Silvermoon). If Halduron, Liadrin, Rommath, and Aethas are any indication, Lor'themar has also, to some extent, re-established the Convocation of Silvermoon, suggesting he may be leaning more toward establishing a permanent parliamentary government once things settle down long enough for him to focus on long-term governance rather than just perpetually extend the regency as a band-aid.
And while there's been rough spots under Garrosh and Sylvanas, by and large every decision Lor'themar's made since accepting the regency has been a net gain for the blood elves without necessarily being a net loss for his allies (whereas Anasterian and Kael'thas were less concerned over how their choices impacted their allies, whom they were fair-weather friends with at best).
So with all this taken into account, why would the blood elves, who have been enjoying a resurgence under Lor'themar's leadership, even if it's not exactly the most thrilling regency to live under, suddenly welcome back a prince who betrayed them for power and favor with the Burning Legion? The same Legion, mind you, that engineered the Scourge that nearly wiped them out as a culture and species. Alleria was permanently exiled and told in no uncertain terms she is not welcome back for something she had no control over; why, then, would someone who acted knowingly and with malice be welcomed back and reinstated as King considering the blood elves' reputation for harboring a grudge?