Most languages have a single implementation that everyone uses. Unless I'm interested in writing a new implementation (never), then I'm going to be interested in comparisons of languages based on their well supported implementations. Otherwise we get into discussions about sufficiently smart compilers (http://c2.com/cgi/wiki?SufficientlySmartCompiler). I get what you're saying, but there are tons of people who are justified in only caring about implementations.
But for the most part those alternative implementations have similar designs and performance characteristics to the most popular implementation. E.g. for Java, all major modern implementations use a JIT, and the language lends itself to a JIT runtime, as virtual-methods-by-default performs poorly when statically compiled.
