Because no one was clearly better than anyone else. It wasn't like Vettel dominated all the races but just suffered bad luck. They all made their own mistakes, none of them had more or less luck than each other.
Vettel simply lucked in the last race (as well as obviously utilised great skills, but even so, lucky that Webber and Alonso were screwed thus), I don't think anyone would have been a better champion out of Hamilton, Alonso, Button or Webber this year, but neither is Vettel the most deserving out of them.
Of course Vettel earned the title and he is a great driver. He would deserve another one. I'm not trying to "devalue" Vettel's title here.
But did he stand head and shoulders above the rest this season to claim this crown? No, no one was clearly ahead this season which is unique to 2010 and possibly 1983.
How many Poles has Vettel turned into victories? Comparing to Schumacher and Senna is premature indeed. Sure they also achieved awesome stats like that, but they also won most of those races.....
I agree no driver was clearly dominant this season, but you can't say no one had more or less luck than each other. Clearly, Vettel has lost by far the most amount of points out of anyone, about 66 in all I believe purely because of mechanical failures, that's excluding his brake failure towards the end of his Q3 lap in Turkey (the permutations of which are incalculable
)
By contrast, Webber hasn't had any mechanical failures that cost him any points, yet he still finished behind Seb.
What impressed me most about Vettel is that even after all the set backs he kept coming back and comprehensively outshone Webber in that all important Q3, who before meeting Vettel was known as a qualifying specialist himself. From Suzuka onwards Seb was utterly flawless, he peaked at the right time and went out and won the championship for himself.
Still if each driver is judged objectively, based on how quick their cars were, how quick they were and how many mistakes they each made, I think you'll find that Vettel was probably only the 3rd or 4th best driver this season, IMO behind both Hamilton and Kubica.
Hamilton can't seem to shake the tag of being 'error prone' or 'overly aggressive' despite him making fewer mistakes than any of the 4 title contenders going into the final race (mistakes of which were smaller and more marginal than the ones his rivals had made) and also the fact that he was still in the reckoning despite driving for the majority of the season in what was the third fastest car and finishing well up on his team mate, who had half the number of DNF's as him. Personally I think 2010 was Hamilton's best year in F1 to date, the first season where he's blended his speed with the calm and error free head that he was missing in '08. Plus I believe he can lay claim to the most impressive drive of the season at Melbourne, cruelly unrewarded due to factors outside his control.
Kubica is rather harder to evaluate as it would be like trying to compare an LMP2 driver at Le Mans against the LMP1 drivers. But what should be said is that he displayed this year that he is a top drawer driver who can marry tremendous speed with error free performances, consistently throughout a season - he's the one the rest of the field worries about getting in a quick car. Yet it's still hard to call his season the best of the lot, when you consider that he was under far less pressure than the title protagonists and had a team mate who is unquantifiable given that it was his debut season in F1. What should be looked at most closely however are his qualifying performances at Monaco, Spa and Suzuka which were the best examples of his brilliance this year.
Alonso was without question the driver of the second half of the season, but I believe when the whole season is taken into account, he actually threw away a lot of points, due to silly unforced errors. Also, when you consider that he lost just 2 points due to unreliability (Malaysia), and yet gained 22 points because of other drivers' misfortune (Vettel at Bahrain, Vettel at Melbourne, Vettel and Hamilton at Barcelona, Vettel at Korea and possibly more if you count Hamilton's gearbox penalty at Suzuka) and that's not counting Hockenheim, but I don't want to go there. Such is the reputation of Alonso though, that his various mistakes are rather swept under the carpet and forgotten about because they don't sit well with peoples' perception of him.
In conclusion, I believe Vettel, Hamilton, Alonso and Webber would all have made worthy champions, just that some would be worthier than others
No wonder Spain's thinking about banning it.
That's quite scary actually - poor camera! 
Hopefully next year Ferrari will supply him with a car more capable of competing toe to toe with RBR and he will just have a more consistent season in general. Regardless, great first season at Ferrari and he definitely showed immense fighting spirit in the second half of the season to get back into the fight, despite facing such uphill battle with a inferior car and such a large points deficit.
All I can say is good luck going into next year, you're going to need it especially if Vettel has the reliability your car did this year.
Getting it done! 
