[0]: https://en.wikipedia.org/wiki/Libyan_financing_in_the_2007_F...
First, this is mostly about things that happened before his election.
The tribunal ruled he did not personally benefit, and he did not directly solicit money to finance his campaign either.
However, some of his closest allies (who would become his ministers later) did the latter. The tribunal could not find any direct proof he was involved but ruled there were enough "converging indications" that he knew and did nothing to stop it.
The tribunal didn't rule he didn't personally benefit. It ruled that he conspired to corrupt the leaders of Lybia to steal money from the Lybian people and fund his electoral campaign. In my book becoming president of France is certainly a "personal benefit". There are numerous factual evidence, documents from Lybia, fund transfers, secret meetings of his closest friends with Abdullah Senussi, who has been convicted to life in prison in France for orchestrating the bombing of UTA flight 772 which resulted in 170 deaths and is also currently investigated for another plane bombing.
The money he got allowed him to spend about twice the allowed amount on his campaign, giving him an unfair advantage in the election. In other words he dealt with terrorists to potentially steal the presidential election. What Sarkozy did is extremely severe, I'd call that high treason. He got far less that he deserved.
Also it's worth mentioning that it is his third conviction. He already got a 2 years and 1 year sentence which were confirmed in appeal in other cases.