It's been over 10 years, I can't believe a C-list actor would turn down a voice gig if offered - just look at Pegg (ummm..."zing"?). I doubt that it's the "official" story, but it makes absolute sense. When it was officially announced that Wittweir was voicing Maul, TFN ran an article explaining why the casting change wasn't much of a surprise. I remember (incorrectly?) the link ran to the public posts between Serafinawicz (?) and Pegg. Given that LFL is (quite rightly - don't **** where you eat, morons) known to completely cut out actors who **** them off by slagging off projects they've been involved with makes sense. Look at Neeson - by all accounts was not too excited about returning under Lucas, but was asked back for the Mortis episode.
Also will say that Pegg as a voice actor blew. There was nothing special in that performance which couldn't have been provided by Tom Kane or another actual voice performer.