Greed to put it simply ....
The original Angry Birds for the ipod touch and iphone was superb. Cheap, simple to play, addictive, frequently updated and original.
They then got greedy with the mighty eagle and such then went down the freemium route for sequels which everyone hates except the game companies and the people getting a cut like Apple and Google.
Sadly most of these game companies are very much a one trick pony, it has happened time and time again where companies like Rovio come from nowhere, get heavily invested in, go to shit then go bust.
Nothing new.