#1) The need for a large ellipse of flat plains with a minimal amount of hazardous rocks/slopes. Landing accuracy is constrained by our knowledge of the Martian atmosphere. Odd features tend to be on the opposite end of the ground hazard spectrum.

#2) Planetary protection. Mars his a Category V target: it has potentially habitable niches, it has an atmosphere able to carry biological contamination anywhere, and we want to return samples from it to Earth. Chances of contamination are much lower when we stay far away from water ice and active gullies. Modern landers and rovers are much more delicate and intricate than the original Vikings and we can't just bake off all the contamination.

#3) Communications. Equatorial plains have a better view of the relay satellites.

#4) Close up, most interesting features on Mars are still a rocky desert. The Phoenix Lander landed on top of permafrost, which was unintended as it was thought to be just another rocky desert.

#5) Odd features are often complex geologically and characterizing the area near the rover may not say much about Mars nor provide useful context for future, more detailed missions.

#6) Perhaps, a tiny bit of backlash from Viking's ambiguous chemical experiments. It's easier to get conclusive science and write solid papers from old, stable parts of Mars.