I actually worked at Sun as a Java architect back in this era, and consulted with the JPL as they were adopting Java.
In the end, the JPL isn't a software shop. The glory is in the hardware end of things. As a consequence, you want something that has a broad pool of talent, tooling, vendors, etc. to draw upon for support. It also was a more natural transition from their extant Fortran & C code.
LISP's marketshare had already declined significantly at that point, and it's primary remaining user base didn't have very similar needs to the JPL. When you're an outlier for a language with small marketshare, that means the tools, skills, vendors, etc. aren't going to be as well suited to what you're trying to achieve.
tl;dr: when you're in a domain where you aren't trying to be quite so innovative, it helps to run with the pack.
In the end, the JPL isn't a software shop. The glory is in the hardware end of things. As a consequence, you want something that has a broad pool of talent, tooling, vendors, etc. to draw upon for support. It also was a more natural transition from their extant Fortran & C code.
LISP's marketshare had already declined significantly at that point, and it's primary remaining user base didn't have very similar needs to the JPL. When you're an outlier for a language with small marketshare, that means the tools, skills, vendors, etc. aren't going to be as well suited to what you're trying to achieve.
tl;dr: when you're in a domain where you aren't trying to be quite so innovative, it helps to run with the pack.