The Altair lunar module was part of the wonderful Constellation program cancelled in 2010; just the Orion spacecraft has been retained which is to deliver a crew into lunar orbit. The new carrier rocket will be the SLS (to replace Ares I and Ares V) but the lunar lander is to be replaced too. Why, as a matter of fact? Why don't they just again aspire to build the Altair LM from the Constellation program, or even go a step farther and rebuild the Apollo LM, just with upgraded instruments and a digital navigation system, as well as a way to match into the SLS stage?
According to Wikipedia, Artemis 3 is to land two astronauts on the surface while the other two crew members stay in orbit or at the lunar gateway station. So the Apollo LM could be reused at first. And if the Altair would be built, all four astronauts could land on the surface.