Is better C++ interop the motivation for Clasp over Julia (which has lispy roots and CLOS-like open multimethods, though not lispy syntax)? Julia is conspicuously missing in the benchmark, but should do pretty well if you don't include JIT/start-up time.
If the question is why not use Julia? The answer is - for several reasons. I started implementing Clasp before Julia was a thing. The Julia language keeps changing and it doesn't have a standard like Common Lisp, C++ and C do. I need tight C++ interoperation and Clasp does C++ interoperation like no other language I've see. Clasp uses C++ exception handling and calling conventions and this allows us to compile and link C++ code with Common Lisp - it's all LLVM under the hood. Clasp stack frames and C++ stack frames are interleaved and try/catch and RAII and Common Lisp unwind-protect and dynamic variable binding work perfectly between C++ and Common Lisp code.