In addition to it being a historically popular implementation, from the MCL days, there was a period of time when SBCL was still pretty rough while CCL was matu

In addition to it being a historically popular implementation, from the MCL days, there was a period of time when SBCL was still pretty rough while CCL was mature and performant. They've swapped places IMO in the past decade or so partially due to the excellent work by the SBCL developers while CCL has been less active over the same timeframe.