Mainly because of the distance to the sun, which makes Mercury a lot warmer and Mars a fair bit colder. Mercury is too hot to have any running water, and Mars is too small to retain an atmosphere.
Earth > Venus > Mercury > Mars
First of all, Mars is a lot closer to Earth than Mercury. The other reason is that since it is further away from the sun, it is not as hot as mercury which makes it easier to send rovers.
The order of Mercury ,Venus and Mars by size is: Mercury ,Mars and Venus
Mars, Mercury, and i think that's it.
Mercury is closer to the sun than mars
Mercury
Mercury is the first planet from the sun, Mars is the fourth.
Mars is only about a third larger than Mercury, having 1.34 times the surface area of Mercury. Mars-6,788 km Mercury-4,880 km
Mercury, Venus, Earth and Mars
Mars
Mercury is warm but Mars is not there could be water on Mars there are ice sheets on the N/S poles of Mars too