A major problem with any racing car kits comes down to licencing agreements (using brand names and logos); coupled with the fact that every season cars change. This creates a lot of work for the kit producers, new box art, decals and instructions at the very least.
And really how many times will you buy a slightly altered kit just because it's this seasons version (WRC even changes decals for many of the individual rallies)? This creates a low volume of sales per kit issue.