Why does Jankovic lost 4/6 4/6 against Vaidisova ?
Why does Ivanovic lost 3/6 1/6 against Golovin ?
Why does Chakvetadze lost 3 match in a row ?
Why do Petrova and Sharapova are injured ?
BECAUSE The calendar is tooo long, players are exhausted ...
Well, that's one explanation.
I'd argue that: Jankovic overplayed this year, and Chakvetadze is suffering mental withdrawal after a letting golden opportunity for a slam final fall apart. Plus, Vaidisova isn't exactly a pushover, and Chakvetadze has only lost to top 20 players since the US Open.
Ivanovic is streaky, and Golovin is hardly a nobody.
Petrova and Sharapova are injured for a variety of reasons, and overplaying isn't the fault of the calendar. If you wanted to, you only have to play 5 tournaments. That's not an awful lot. I'd argue it's because their bodies aren't built properly for the stress they put on it with their games. They play too physically for their body types, and they should know better than to play heavy schedules with those problems.
Petrova has been over taxing her body for four years now, and Sharapova is the victim of her limitations; her attempts to become more well rounded as a player have decreased the number of hours she can play. If she strikes the right balance, she can still be number one.
Henin had the same problem in '04 after she bulked up for the '03 season and played around 20 tournaments; now, she plays very selectively, and paces her year extremely shrewdly. The Williams do that as well, although the technical deficiency of their game leads to injuries regardless. That doesn't mean they (Sharapova, Venus and Serena, Justine, etc.) are bad players, just that they need to find the right balance between fitness, power, all-around ability, and scheduling. There's not much that the tour can do about it.