Simple. It was the easiest to land on. Titan has an atmosphere, which makes landing there quite a bit easier than landing on Europa, which does not.
In addition, Europa has only been known as an object of interest since Galileo, which was the last mission that even had a chance of sending a lander there. It was suspected as an object in the Voyager flyby, but it was much less sure until recently. Cassini was under works, along with the mission to Titan.
Titan is as interesting of an object as Europa, and has been known to be an object of such interest even well before the space age, due to it's atmosphere of hydrocarbons. Titan, Europa, Enceladus, Triton, and slowly Pluto are emerging as the most interesting objects in the outer Solar System, from the perspective of potentially having life, or at least the building blocks. While Europa is now known to be the most promising candidate, it took scientists a while to realize that, and there hasn't been time to send a mission there.
Lastly, there is the issue of radiation. Europa, being so close to Jupiter, is the subject of powerful radiation, similar to the Van Allen belts. That makes it even more difficult to manage a Europa lander.