Just because it worked in Paris, didn’t necessarily mean it would work here (and I’m not going to get into the same arguments as we’ve had before re. Parisians living in the centre of town and the bikes not moving in the tidal pattern that we see). London is a different city.
Also, how much revenue do JCDecaux generate? I’d wager they’re turning a profit from that contract. Barclays get to bleat about their association with the scheme, but the direct benefits are very difficult to quantify. It’s brand awareness for sure, but it’s not actually selling a specific product.
Boris’ statement about providing the whole thing for free suggests that they wanted a JCDecaux-style deal, but it wasn’t forthcoming. Perhaps JCDecaux (or whoever) also realised that London is a very different city to Paris.