Remember that the block hash must be quickly verifiable. This means that calculating single hash should be as fast as possible on CPU. Otherwise only miners would be able to verify blocks.
With the RHA algorithm the miners would still need to do zillions of hash calculations while verification is only one RHA calculation. Sure, RHA is hugely more computationally demanding than say plain SHA-256, but verification can be done within a second with a CPU I guess.
Or do you mean ordinary Bitcoin nodes need to do many block verifications per second? That could be problematic with RHA.