Because it totally destroyed the balance of the game that existed before LoR was out. It was designed to be a small DLC to improve the internal mechanics of the Byzantine Empire and let a good player form Rome, thereby giving a few bonuses and letting you mend the Great Schism. By giving the Imperial Reconquest CB to the Byzantines straight off the bat, there was no real gameplay reason to form Rome (just a new ruler title) and it also meant the AI would frequently conquer Italy, the Middle East, chunks of southern France and -- in theory, though admittedly I hadn't seen it in my handful of 1.07 games -- England, which is about as absurd a game concept as I can imagine. There is simply no genuinely conceivable reason for the Byzantines to have a blank check CB against England or France from the 11th century onwards.
What we have now is a system whereby forming the Roman Empire gives you a solid, genuine bonus in the form of the Imperial Reconquest CB, and the Byzantine AI now behaves much more realistically as its conquests (aside from via any claims it fabricates or inherits) are is now limited to its de jure borders.
It's also much harder, which is exactly how it should be, and presumably the reason why the same people who previously could re-conquer half the map as a restored Rome by 1150 or 1200 are now complaining.
