There could be several reasons...
IANAL - but I'm married to one...so I know a BIT about this.
1) He may have been accused of rape and found not guilty. If that's the case, it's certainly true that the previous case should have nothing to do with it.
2) In general a previous conviction has nothing to do with the current one. You've got to PROVE the case - and saying "well, look, it must have been him, he's done it all those other times" isn't proof that he did it this time. Having said that, a prosecutor CAN apply to get "previous bad character" in as supporting evidence - but there has to be a good justification.
Previous convictions ARE taken into account when sentencing, of course.