For Many Reasons
1) You can't make them habitable without adversely affecting much of Earth.
2) Rock is more stable than ice.
3) The polar day is substantially different from a normal Earth day.
4) Solar energy is missing for 1/2 the year like it's missing half the month on the Moon.
5) It's still Earth and offers none of the advantages of an independent civilization including the necessity of innovation.
6) We do send people there to do extended research. They don't try to be self sufficient because its easier to ship supplies. Not true on Mars. Only once self sufficiency is established to a significant level does colonization become practical.