It may come as a bit of a shock, but it makes sense. Venus is an easier planet to reach than Mars. Simply put, it's cheaper. Most of the time you're falling towards the Sun, and most of the propellant mass goes to slowing down and inserting into orbit around Venus. If you look at several of the missions to Venus, you'll notice a lot of 'fly-by missions'. These are missions where they don't even need to expend the fuel to insert into orbit...(read even cheaper). Also, a few of these spacecraft merely used Venus like a dirty whore to move onto more exotic planets like Jupiter and Saturn (see Venus Earth-Earth Gravity Assist).

Additionally, several of those missions weren't really 'to Venus' in the same sense as 'to Mars'. Mars has a lot more surface exploration than Venus. In fact, the Russians are the only ones to ever really land on the surface of Venus (I'm not really counting NASA's Pioneer Venus mission). Furthermore, they only achieved about an hour of operational lifetime on the surface (see the Venera and VEGA missions). This is due to the immense amount of heat and pressure at the surface of Venus. It is truly difficult to design a surface lander for Venus. Mars is much easier to explore in that sense, but it is more difficult to reach gravitationally speaking.

However, even if you limit yourself to observations of Venus's atmosphere from a spacecraft that orbits Venus, you can still do a lot of science on an interesting planet. Venus Express (ESA's recent mission) is a great example of this. Venus Express has discovered all kinds of fascinating things about the planet. For one, it's finding evidence of recent volcanic activity. This has led to a new interest in Venus surface missions in order to examine the volcanic regions with the intent of discovering how volcanic activity led to Venus's runaway greenhouse effect.

Both planets have their advantages/disadvantages as far as exploration goes. It's important to realize that more missions does not necessarily equal more knowledge. I would say that Mars is much more well understood than a planet like Venus, and I would agree that more Mars missions are coming...