Mercury reason #1: Instead of using orbiters at Mercury, Robert Forward's Statite's might be used to hover such that they are constantly in the sky above the more useful polar regions.
Mercury reason #2: Some of the people commenting above wondered what need there would be to have orbiters simultaneously around each planet. One reason involves climate change here on Earth. I'm not a climatologist, but I imagine that it would be useful to understand the sun's effect on the atmospheres of each of the planets, so that we can better distinguish between the effects of the sun, and other terrestrial causes of climate change ("Beware the beast man, for he will make a desert out of his home and yours....") I'm thinking of this system-wide monitoring as taking place in a moderately far-off future where future tech makes things a bit easier. But Mercury, being atmospherically-challenged, might be the one planet where we wouldn't monitor the sun's effect - so Mercury would be the spoiler, and we'd only have 7 of 8. On the other hand, as Pluto's atmosphere resurrects itself (the atmosphere comes and goes at different points in its orbit), we might be monitoring its atmosphere instead, so we'd still have 8 orbiters around objects historically thought of as planets. And maybe we'd want to monitor the atmosphere of Titan, and maybe even Triton's too.
Blame Mercury
Mercury reason #1: Instead of using orbiters at Mercury, Robert Forward's Statite's might be used to hover such that they are constantly in the sky above the more useful polar regions.
Mercury reason #2: Some of the people commenting above wondered what need there would be to have orbiters simultaneously around each planet. One reason involves climate change here on Earth. I'm not a climatologist, but I imagine that it would be useful to understand the sun's effect on the atmospheres of each of the planets, so that we can better distinguish between the effects of the sun, and other terrestrial causes of climate change ("Beware the beast man, for he will make a desert out of his home and yours....") I'm thinking of this system-wide monitoring as taking place in a moderately far-off future where future tech makes things a bit easier.
But Mercury, being atmospherically-challenged, might be the one planet where we wouldn't monitor the sun's effect - so Mercury would be the spoiler, and we'd only have 7 of 8. On the other hand, as Pluto's atmosphere resurrects itself (the atmosphere comes and goes at different points in its orbit), we might be monitoring its atmosphere instead, so we'd still have 8 orbiters around objects historically thought of as planets. And maybe we'd want to monitor the atmosphere of Titan, and maybe even Triton's too.