Nick, the pre-2017 WRC rules are basically nothing more than slightly modified S2000 rules. Those were introduced around 2005. The very first S2000 actually had clutchpack-type center differential (Toyota Corolla (RSA), VW Polo (RSA) and Fiat Punto). Peugeot in 2007 was the first one to bring transmission without center diff at all. I believe these were the main reason why not to use it:
- for naturally aspirated S2000 engine the power lost in the center diff was not negligible
- such simple clutchpack brought no significant advantage in handling performance over the variant without center diff at all
- it was an additional weight; in that time the weight limit for S2000 cars was 1100 kg and I know from Barum scrutineering that the limit was totally not achievable with the given rules; i.e. the less weight the better

All other following cars had no center diff at all even though for example Škoda used both variants in testing for comparison. Later even Fiat changed transmission in the Punto for one without center diff.

Honestly I thought that starting with the new WRC rules (2011) the center diff was explicitly banned.