Last time Šumava was in February was on snow was six years a go. No-one has same car as in that time. In those years there were two winter rallies in the championship, so having the suspension and tyres made sense. However in Šumava there were big problems with forbidden use of studs in CZ, so there was only Jänner and that was the problem. One rally for which You need special equipment and moreover abroad (I know it's just behind the border but for many sponsors it doesn't matter).
Let's see it from view of let's say Peták or Odložilík. Gravel suspension for S2000 costs maybe 40-50 thousand Euro and for what is it good for them besides one event? The other problem is that large majority of drivers doesn't have sponsorship deals done in first week of the year...
As I said those drivers who wants to drive Jänner would drive it anyway regardless if it is in the championship or not. That is let's say 20 of them but that's far from majority.