2 reasons:
1) Wenger saw Djourou, Squillaci and Koscielny as good centre backs when they are in fact sh*t
2) Vermaelen was like a new signing and Wenger never took into consideration the fact the guy was out for a year and probably need to be brought along very slowly, as after such a long period players tend to be more prone to further injuries - a la Eduardo
There's another reason too to be fair, we're cheap and wanted to make a profit this summer, that's why we also bought the German barge, he was relatively cheap.