Time travel 8 years ago, Arthas and his death knights get destroyed by the Argent Dawn+Ashbringer at LHC. Present day, the DKs think its a good idea to invade LHC again, but this time its not just the AD, its all the paladin orders united under the Silver Hand. Granted most of the paladins are off at Netherlight Temple at this time, but Arthas (and by extension, now Mograine) knew LHC was holy ground and that they could not win a battle at it.
How does it make any sense at all for DKs to even think about bringing Tirion back given past events at the LHC?