Because after you destoyed the Kubel the counter unit becomes completly useless and destroys itself immediatly. kappa
Because M3 and UC are soooo part of meta atm.
M20 is excused and decent piece of light armor.
And did I mention that countering kubel gives OKW 5 fuel as a compensation, so you're not only delaying your tech, but speeding up your opponents as well?
Fuel free kubel is cancerous, it could work for Ost, but its completely cancerous for OKW atm, menpower cost can be lower, but it shouldn't be fuel free.