zkSnarks - why isn't PCD more widely used?
There are two types of overhead in PCD systems:


Overhead of proof verification: Computing the pairings necessary to verify prior proofs is expensive, and adds overhead unrelated to the statement you're trying to prove.
Overhead of predicates: In the Scalable SNARKs paper, each recursion level verifies the execution of the vnTimyRAM CPU. This involves verifying the CPU execution itself, as well as authenticating that the memory accessed during this step is correct. Checking CPU execution is cheap; it requires roughly a thousand gates. Memory checking, however, is very expensive; each access requires verifying a Merkle tree path. While the authors try to mitigate via the efficient Ajtai hash function, the overhead due to this check is still the largest in the circuit.


Together, these overheads are much more expensive per cycle than other approaches for verifying RAM computations; the primary reason to use recursive SNARKs is for the lower memory consumption.