Why are only the poles of Mars frozen, despite too cold temperatures on lower latitudes? Actually the entire red planet is very cold, cold enough for ice and frost to form not only on the poles. Temperature rises above 0 deg Celsius in summer only. Why then are just the poles frozen, and not lower latitudes during most of the Martian year? Is it because most of these areas perhaps aren't within the triple point of water, unlike the poles?
Related questions on StackExchange

Mars' atmosphere and the triple point of water
Which “exotic salt” can lower water's freezing point by 70 °C?
Images of Mars Phoenix Lander showing what's happened after more cycles of winter ice build-up?
What's the scientific evidence of water for return trip methalox on Mars?
Would snow fall on the surface of Mars because of uncovering the ice deposit in Utopia Planitia?
What is the warmest place on Mars?
Just like a cold winters night on Earth does not guarantee snow, so does a cold Mars not guarantee ice everywhere.
In order for ice or snow to form, the moisture that is later solidifying has to be available at the cold location. Moisture in this case is $CO_2$ with traces of water, leading to an ice cap made of $CO_2$-ice, with a ring of $H_2O$-ice 'trailing' around its outer radius.
The atmosphere is in fact only cold and dense enough for $CO_2$ to solidify at the poles, so no mystery there. Otherwise the entire atmosphere would condense out into a thin layer of $CO_2$-ice. To understand the formation of the $H_2O$ ice, we have to take into account how the water is transported inside the martian atmosphere.
Mars possesses only one global circulation cell (see early modelling on this by Haberle et al., (1993), and a recent review on the martian climate for the interested reader by Read et al., (2015)).

((Figure from Haberle et al.,(2015))
The reason for the single cell is a combination of Mars' small size and rotation period, as outlined in this answer. The effect of the single cell is that the condensable moisture is only transported essentially pole-to-pole directly, with only small variations of ~10-15 degrees latitude during the martian year.
Hence, no other location on Mars than the poles can accumulate a significant enough vapour pressure in order to form ice.