The variable people aren't considering herein is nationalism, which as we have seen from real-world history can take on numerous forms and magnitudes.
It's perfectly reasonable to suspect that many groupings of elves, regardless of the many slights of the wars and having lived outside the core holdings of their kingdom for a prolonged period, would have either grown apart from their social/cultural/economical ties to their kingdom or else willingly associated themselves with third-party local, regional and national entities.
To expect that all elves were loyal to a solitary authority, prior to some schismatic event that was close to being farcical, is somewhat naive. Their spectrum of politics is very likely dynamic, even if neither game nor book can fully establish this as a basic truth. The brief version:
--A majority of elves placed most of their loyalties with Quel'thalas.
--A minority of elves placed most of their loyalties with Dalaran.
--A minority of elves might have placed most of their loyalties with Lordaeron.
--A minority of elves might have placed most of their loyalties with none of the above.Quel'thalas was no longer part of the Alliance (nor was Stromgarde) at the time. They had no obligation to come to the aid of Dalaran. Furthermore, Quel'thalas was destroyed before Dalaran- Archimonde was only summoned after Arthas raised Kel'thuzad in the Sunwell.
This is all well and good, to be sure, but for an elf that spent perhaps 5% of their considerable lifetime in Quel'thalas and the other 95% in Dalaran there isn't really a reason to side with the former over the latter. This goes both ways and is perfectly understandable in both cases.
This thread has been considering nationalism. If we consider that Anasterian Sunstrider, who was at the very LEAST a bit over 2,220 years old at his death, was considered to be of advanced age... it's entirely possible and likely that many high elves were born in or spent the majority of their lives in Dalaran. Dalaran didn't pop into existence recently, it's got at least a few millennia of history under its belt.