there are multiple factors that have resulted in a decline in this tournaments guarantee, let me see if I can type out what they all are!!
firstly, since it is the summer months, a lot of the people who would play this MTT will be out having BBQ's and partying.
since PS increased the rake by 20%, there will be quite a few players that will not be happy about this and therefore will stop playing this game.
also continuing on talking about rake increases, there will be a lot of players who will have stopped playing the sattelites due to the rake increases.
again continuing on with the rake increases, there a lot of players who have completely left pokerstars because of the rake increases.
another thing I would like to add is if pokerstars decided to put a 500k guarantee on next weeks Sunday Storm, the guarantee would most definitely be hit.... why you may ask.... because a lot of players see this guarantee as juicy and will fire the MTT, so deep deep down in this decline of guarantees pokerstars has no-one to blame but themselves.
for instance, look at these "common cents" right now, why do you think they are able to get a $10k prizepool on a 99c buy in? because they have enticed a lot of people in by putting a $10k guarantee sticker on the mtt.
if they did that everyday, obviously we would all pay 99c to play in a $10k guarantee mtt, why don't they? who knows....