So, I generally find meme posts really annoying. As Umair Haque said, "This isn't 'content', in any meaningful sense of the word. It's more like 'emptent' -- stuff that fills us up, only to leave us empty." It's a meme-industrial complex.
They're brain-candy -- a brief factoid, no context, no real insight, and far too frequently, the ensuing discussion (go check the original thread) is ... less than edifying. They're the equivalent of those lists of facts that used to appear on cereal boxes, or in the Sunday Funnies, or listicles on those now-defunct weaponized viral clickbait sites. Oh well, one can dream....
But if you look at the claim, it turns out that there are reasons that can be teased out of this which make the factoid less surprising.
Mars is Distinct
The reasons for the high level of robotic activity on Mars are largely due to its own characteristics:
It's relatively far from the Sun. Unlike Mercury, this makes reaching and orbiting it fairly viable.
It has an atmosphere (like Venus, Earth, and the gas giants), but not so much of one that any probe would likely be crushed (as on Venus and the gas giants). This allows aerobraking, vastly reducing rocket fuel requirements. Landing on Mercury, the Moon, or asteroids would require landing rockets.
And robots, once landed, frequently survive.
It has a solid surface, unlike the gas giants. It's one of only 3 non-Earth planets to have one, though several non-planet bodies offer landing opportunities.
It is cool. Both Mercury (427 °C / 801 °F) and Venus (462 °C / 863 °F) are hot enough to make engineering durable probes difficult.
It's pretty much a case of the lamplight syndrome. We're looking at Mars so much because that's where the light is, or at least, that's where we can get to relatively easily.
We've sent flybys past Mercury, but didn't manage to send an orbiter to the planet, the Messenger mission, until 2011. It ended by intentionally crashing the probe into the planet on April 30, 2015. Orbiting Mercury is hard, and the orbits themselves are unstable.
We've had orbiters around Venus, which is fairly doable, but the landers don't last long due to surface conditions. We've sent robots to Venus, but they all died (as anticipated).
We've got lots of landers on the Moon, and a few orbiters. It's not a planet though.
The gas giants: Jupiter, Saturn, Neptune, and Uranus, have seen flybys and some orbiters, but they're hard to get to, the atmospheres would crush any probe, and there's no solid surface.
Pluto's no longer considered a planet, and is tremendously far from Earth. We've sent a probe which will fly by it on July 14, 2015, New Horizons. It will be travelling at a relative speed of 49,600 km/h / 30,800 mph, which means that in the four hours it will take signals from the probe to reach Earth, New Horizons will have travelled 198,400 km (123,280 miles) past Pluto. That's roughly half the distance from the Earth to the Moon.
Or in other words, it won't be hanging 'round long.
As Pluto also has little atmosphere for aerobraking, a soft landing would be exceptionally difficult as well.
Inhabitants of exo-solar planets are undetermined.
And of course, Earth has other inhabitants.
So if you want to 1) land 2) a robot that 3) will survive on 4) a planet that 5) doesn't have other inhabitants, Mars and Mercury are your only options. And of the two, Mars is hell of a lot easier.
- I swiped that shamelessly from either Imgur or reddit.