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.
Mars special regions as defined by The Committee on Space Research (COSPAR) means that planetary protection is the main reason that the US and China have/will avoid the polar regions. If forward contamination occurred, it could wipe out any indigenous organism under the mixed ice surfaces. How we proceed in the investigation of these locations without cross contamination is not yet known, but the interim protocol is defined as being If a hard landing risks biological contamination of a special region, then the whole lander system must be sterilized to COSPAR category IVc.
NB I think it will eventually be imperative to visit the poles, but at present the risks seem too great to take. Also the curious phenomenon of seasonal Martian geysers coming from large spiderlike features, apparently spewing sediment that give rise to dark dune spots, have been proposed by some researchers as a sign of biological activity. https://en.wikipedia.org/wiki/Geysers_on_Mars#Hypothetical_biological_origin
For some good pictures of these dunes see, https://www.npr.org/sections/krulwich/2012/10/02/162147810/are-those-spidery-black-things-on-mars-dangerous-yup?t=1619264032435
See NASA planetary protection web page. https://sma.nasa.gov/sma-disciplines/planetary-protection/
Also see COSPAR policy regarding space probes and landers. https://cosparhq.cnes.fr/assets/uploads/2019/12/PPPolicyDecember-2017.pdf
The poles are very cold, so thermal protection requirements are greater.
The poles reduce the potential utility of solar power, thus effectively forcing the lander to use non-solar power, which is heavier and more expensive.
But most of all: The poles are surfaced in various ices.
Ices that accumulate, and sublimate away, and accumulate again. The surface texture, and structure, and loadbearing ability are all unknown and virtually unknowable without extensive testing.
There is a similar climate on Earth, where ice falls as snow, compacts, and then sublimates away without ever melting to liquid water. The Penitentes, in the high Andes mountains of Chile.
Aside from the reasons given above, there may be some orbital mechanical issues, since a landing would likely be preceded by an orbital insertion and the lander itself may be accompanied by an orbiter. (Moon missions commonly employ such a combination.)
First, insertion into Martian orbit does not give a polar orbit for free. A spaceship launched from Earth would arrive along the ecliptic plane which, as pointed out elsewhere, has about the same angular displacement from the polar ais as does Earth. Reorienting the orbit comes with an additional cost on $\Delta v$, which would be similar in magnitude to the orbital velocity itself.
Second, Mars has very uneven terrain between its northern and southern hemispheres, which contributes to a $J_2$ value nearly double that of Earth. The orbital perturbations caused by this nonsphericity would be maximized by using a polar orbit.