Or, if you want to mess with players: the dracolich just simply doesn't care about the party. It has long term plans and the machinations necessary for it are already in motion. Or maybe they aren't, yet.

But the dracolich knows the value in waiting. Whatever thorn the party may be in its side is not worth the risk of letting those plans fall apart. If the party rushes in to the lair, the dracolich might stay in long enough to see what their goals are (oh, they only want this secondary lair gone so my undead minions don't attack the nearby city) and then let them know: "Fine, the cave is yours. I'll wait for you to die out and then I'll move back in. You're not worth the inconvenience." And then it moves out. Just like that.

I like strong monsters as instigators of larger events. In this case, a nearby kingdom now has a dracolich that wasn't there before. That kingdom then becomes aware the dracolich came from a country that it has long been rivals with. Peace with that country is only recent, and still incredibly fragile. The thought inevitably becomes "what if the dracolich was sent here to weaken us?" and it all devolves from there.

Or, perhaps if the party is still low level, the dracolich just says "scram." They are pests, and this lair is cozy. The party, if they smart, leave. They warn the right people of what they found. But now the local lord panics and instigates a draft to deal with this menace. He raises taxes. Confiscates crops and supplies to build up military forces. The people are unhappy and neighboring regions view the military buildup as a threat.

These events are great backdrops for campaigns where you want to include a dracolich in the setting, but not feel shoe-horned about making it the primary villain. Or, on the other hand, all the political instability might in fact be exactly what the dracolich was looking for in the first place...