Why do you s'pose there's no more production air-cooled cars? Well, all the same reasons are also applicable to motorcycle engines. Emissions, noise, power output per unit of space and weight, and durability all favor liquid cooling. (Durability is better because the tighter temperature control allows parts to be made with closer tolerances - this more than offsets any complexity/probability of failure issues of the waterpump, thermostat, and radiator.)
I know folks who race Buells in a class that we have for them here in Canada. When you start pushing the limits, they say it is tricky to set up the suspension to get them to work right. The H-D engine and tranny force many compromises in the design. For hooligan-bikes, more relevant for street use, they are awesome.