Might have been because of prohibitive cost and super late arrival, not to mention sub par doctrine with limited options.
Now, about that mobile defense osttruppen call-in, which is being used all the time...
Still, the few games i played it looked more that people choosed it for the puma to fill the role of light early reliable AT, not for the osttruppen. But with my limited experience on 1 vs1 i cant really say if its picked for the osttruppen