Which goes to show that this stuff isn't easy to do.
InSight was selected because it was, among other things, the lowest technological risk. Certainly the spacecraft was proven, so that was low risk. The instruments were obviously higher risk.
And although there have been a few people saying that NASA should have picked one of the other competitors (everybody's favorite seems to be TiME), keep in mind that both of those spacecraft were higher risk than InSight. Entirely new spacecraft compared to InSight's lander. In addition, both would have required ASRG power sources, and shortly after the Discovery selection NASA announced that the ASRG program was experiencing significant cost overruns and running into technology hurdles. So either one of those programs might have gotten into even more trouble than InSight, at higher cost.