They picked some of them because they were original cast members (Colm, Alun Armstrong, Judy Kuhn, Michael Maguire) and the others from mainly British productions. Frances Ruffelle was unavailable, so they asked Lea Salonga because she became quite famous in the part for having a great voice and for being the first Phillippine Eponine and such. I don't know quite why they chose Philip- probably since he was popular with Australian and British fans, plus fans of the CSR (plus he was on the Manchester highlights). While I don't think the people they chose were the "best" (how can you say ANYONE is "the best" for sure?), they are pretty good. I sold my cast recording of it because, like you said, it's expensive (I made like $20 selling it used), and it's better to WATCH it. Plus the audience gets annoying to me on audio, especially when they cover up Jenny's lines in Master of the House.
"I have a fever, and the only prescription is more cowbell!"