The reason it's taking so long is because of the gravity assists in its trajectory; that saves on bulk of rocket.

The first spacecraft to go to Mercury, Mariner 10, got a gravity assist from a flyby of Venus; that spacecraft flew by Mercury three times.

MESSENGER is getting several. It already got one from the Earth, and it will get two from Venus and three from Mercury.

As to the ESA's upcoming Bepi Colombo mission, it will also be using gravity assists from the Earth, the Moon, and Mercury, though I have not been able to find many details on that. That may be why it is expected to take 4.5 years to travel to Mercury.