Very simple.

Lack of oxygen there.

Ozone is formed in the stratosphere by the reaction (photodissociation of oxygen)

O2+ O --> O3.

If not much oxygen --> not much ozone.

the short and correct answer was already given by Alain Robichaud above. The longer one is linked to energy required to split 2 CO2 molecules to create free oxygen needed for ozone formation. If one writes an expression like 2xdeltaHf(CO2)+hv=deltaHf(C2O)+deltaHf(O3) where deltaHf is the enthalpy of formation for a given molecule, which is a table value, one will find that the energy hv needed for this reaction to happen corresponds to short UV (~100nm for the whole process, 107nm for CO2->C+O2), plus the probability of having 2 split CO2 molecules in the vicinity of each other is small.

In other words, this process is very energetically unfavorable for Martian atmosphere.

Another factor contributing to low ozone concentrations is the escape of oxygen from the atmosphere. The available Oxygen for ozone generation does not escape from the atmosphere of Earth whereas for Mars, the photochemical escape of oxygen is significant. 1) Less oxygen and 2) The escape rate of oxygen from the atmosphere of Mars are 2 important factors for low ozone concentrations compared to Earth.