Since the Martian poles are water ice caps, belong to the most likely locations on Mars to host life, and the north pole being the most likely place for a crewed mission, it seems reasonable that one should send a lander to one of the polar caps. However there were never missions to the polar caps. Closest were the Phoenix lander which landed close to but not on the north polar ice cap, and two failed probes that crashed near the south pole. What's the reason there are no probes being sent to the polar caps? Is it due to orbital issues? Is one afraid that the probes might sink in Martian snow or due to other possible weather conditions there? As per Wikipedia there's currently no planned polar lander.
The polar ice caps of Mars are more challenging for a number of reasons. Mars has a similar tilt to Earth, which means that the polar regions have some of the same problems. The two main problems are extreme cold and very long nights over the winters. These together mean that there is a limited time when such missions could take place.
For the last 20 years, most of the US landers have been rovers, with the exception of two missions. One of these explored the northern areas, and the other explored the ground beneath. Sending a rover to the poles is pretty much pointless, it would only survive a few months, defeating the point of a rover.
No crew mission will be sent to the poles, at least for the early missions. The maximum latitude for a human settlement is around 40 degrees before the Martian winters become a huge challenge. The most likely sites are those with ice just below the surface, as the Phoenix mission had.
Bottom line, the poles force a short term mission, and there isn't a compelling enough reason to visit them at this time.