In my experience, its almost impossible to get a 2nd consecutive season in a top team.
I am in season 6 of 7 for career mode. I have won the WDC 3 times (seasons 3,4,5)
My season history is as follows: Virgin, Sauber, McLaren, RBR, McLaren and now Ferrari
Even after winning the championship and clearly beating my rival, I did not get offered a contract renewal from McLaren or RBR - yet Virgin and Sauber have. Whats more, the top teams rarely offer contracts mid-season, which is probably where the issue occurs.
However, I think it also has something to do with your contract objectives - you have to meet a certain percentage of them to get a 2nd season offer. Based on some rough calculations, I think its about 80% (combination of qualify position, race results AND final championship place - 37 in total).
This might explain why I get contract offers before the end of the season with Virgin/Sauber every year, but haven't got one even at the end of season with the higher ones.
I am playing on expert though, so maybe i'm just not performing consistently enough - I can meet 90% of race objectives, but only about 60% of qualifying ones.