I believe they said it was out of respect for King Anastarian. Even Kael'Thas was considered "Prince."
I'd be curious if they addressed this if the blood elves ever meet Kael or Anastarian in the shadowlands.
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I had to google that.
https://wow.gamepedia.com/Grand_Magister_Rommath Yeah it's a bit "pot calling the kettle black" to be investigating fel cults given the blood elven history. Still, back then it was out of desperation towards survival, and ended horribly for them. Now they have the sunwell and no such excuses, especially when the burning legion is literally on their doorstep killing people and they still have the gall to be like "hmmm, this fel stuff is pretty nice, we should totally take in more of this energy. What's the worst that could happen?"