I would think that the reasons for "why not Scheme" would include the fact that CL has more existing libraries, better built-in support for multidimensional arr

I would think that the reasons for "why not Scheme" would include the fact that CL has more existing libraries, better built-in support for multidimensional arrays, efficient low-level code (now even with SIMD on SBCL using SB-SIMD), application-controlled JIT compilation of specialized code, almost guaranteed support for native multithreading, etc.