CLASP may be newer, but it is hardly more 'modern'. There is only limited adoption of CLASP in the Lisp community, because its advantage (excellent integration into C++) is only interesting for a small group of users. At the same time its disadvantages (complex implementation, much slower than SBCL in both development and application, less compiler sophistication for the Lisp user, ...) limit its adoption. CLASP is a special purpose CL with the aim to easily interface to large libraries of C++ code. It maybe be newer and also a great piece of engineering, but it is no competitor to faster implementations. SBCL can compile itself in roughly a minute on a current machine - something like CLASP takes a lot longer. With SBCL we'll have already the much faster and more responsive implementation.