Because Illusions of Grandeur had already been in print for years without causing any problems. WotC ignored this (well, forgot, but that's not the point) when Donate was printed.
If Donate had been printed first, then Illusions would have been the problem card.
The combo may rely on Illusions of Grandeur, but it also relies on Donate, so what cards are relied upon isn't really important...
The synergies are in a way obvious, but in another way rather balanced. The combo would be less than amazing if Donate costed more, or if Illusions didn't allow for massive lifegain to fuel Necropotence, or if Necropotence couldn't prevent a decking (I've had zero cards in my library several times with this deck, especially since it uses Demonic Consultation). These cards together have powerful synergy, but any one of them being removed causes the others to be much less amazing (although Necropotence is still broken).
You could really then call any of the three "the problem card." I suppose there are three ways of looking at it...
-Necropotence is the problem card because it is broken.
-Illusions is the problem card because it is crap.
-Donate is the problem card because it forged a synergy between two previously unsynergistic cards.
I think the first perspective, although having some merit, is not the best one. Good cards are what makes the game fun to play. Printing cards that work very well with the good cards and then blaming the good cards is bad for the game (at least for any constructed formats).
The second perspective is in this case at least my favorite for deciding which card should be banned if one of the cards needed to be. If Rector-Donate decks somehow became dominant, I would certainly be saying that Illusions should be restricted, rather than Academy Rector or Donate.
But one must still keep in mind that Illusions of Grandeur existed peacefully since Ice Age, being consistently nothing more than a crap rare. Its biggest contribution to the game was inspiring Delusions of Grandeur in Urza's Legacy. This new version was actually usable being that it still gave a massive swing in life, but traded the sheer size of the lifegain for a lack of a cumulative upkeep. This was better--until the next set came out. Illusions of Grandeur didn't magically make itself good, it was made good by the printing of Donate.