We do Venus flybys (flybies?), but not Mercury.
Just a guess is that it’s so close to the sun that the radiation would damage a spacecraft. Is that all or is there something more? After all, the little booger is trucking along at something like 30 miles per second, which would be a pretty good boost.
(I’m speaking only of probes sent to the outer planets, not missions to forgotten Mercury itself.)
I know you didn’t ask, but…
Mariner 10 photos. “More like the Moon than the Moon itself.”
Since I’m here…
My first guess would be electromagnetic interference from Sol potentially interrupting communications.
My second guess would be that Mercury is a very small planet, and therefore might not offer the same slingshot advantages that Venus has.
My third guess is that by dipping that deep into Sol’s gravity well might actually make a Mercury slingshot incidental at best to the total energy imparted on the craft, and therefore unneccesary.
But those, unfortunately, are just guesses.
Slight hijack, but here’s a planned Mercury orbiter, MESSENGER. (That’s NASA shouting, not me: MErcury Surface, Space ENvironment, GEochemistry, and Ranging)
Sofa King pretty much hit the objections. The largest problem is simply heat. Designing a spacecraft to operate in a temperature range of -200 to 300[sup]o[/sup]F is tricky. Flybys are finicky; you can’t just batten down the hatches and coast through 'em in safe mode. The spacecraft has to be awake, talking to Earth, and firing thrusters–then it has to be able to do the same thing when it gets out to the icy blackness of the outer solar system.
Interesting link. Especially the part says it will take 5 years to achieve orbit around Mercury. It doesn’t take that long to get to Mars (yeah, I know the reasons).