WHBM
The short answer is "the weather".
The North Atlantic Tracks vary according to the wind. The arrival times of the flights on those tracks vary according to the wind.
The North Atlantic traffic flow contains a large number of flights all of which have a significant impact on their aerodromes of destination. They also have a significant impact on the sectors they fly through.
This traffic cannot be slotted, it is coming whether you like it or not. That means if they fill up a sector all you domestic or internal European flights get what's left.
That scenario varies from day to day, can only be planned as far ahead as an accurate forecast can be given (i.e. day minus one) and the North Atlantic tracks can be published.
Now, chuck into that a few CBs, a few ATC staff shortages, a few ATC technical problems, environmental restrictions, taxiway closures, flight checkers, fog, snow or what have you and you can see that even the best laid plan is going to be out of date before lunch time on the day of operation.
That completely ignores the airlines' own operational problems. Who ever heard of a train waiting for a missing passenger? Or offloading all the bags. What train has to satisfy as many safety checks as an aircraft pre-flight?
I am happy to answer all the comments above if those with a genuine gripe want to mail me directly.
Compared to life pre-CFMU though, the situation is undeniably better.
And thank you to all contributors to this thread who have posted words of support for FMD. Its a sh!t job but very occasionally rewarding (about the 30th of every month actually!).