Maybe because they would feel they have a duty to stop Sylvanas in whatever she's doing instead of just jumping into the afterlife in Bastion and transforming to a spirit healer?
I don't know the storyline of allied race DK intro, but I'd imagine Bolvar sells it to them as "Sylvanas is breaking the world and afterworld apart, we need to stop her, stop being dead and useless, stand up and fight".
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Nah the most confusing one is Worgens being supposedly immune to resurrection - humans willingly became Worgen in Silverpine to deny Forsaken the possibility of raising them as one of theirs. And then there are Worgen DKs because apparently "it's a more powerful version of raise". Lol wtf, especially since now we've been told that Sylvanas had special powers since the pact with the Jailer done after defeating of Arthas (so since pre-Cata).