SSAS 2000 vs 2008 Cube processing times
Performance usually improves from version to version of SQL Server.  SQL Server 2000's Analysis Services was nowhere near quick.

As to why - Microsoft doesn't give us the source code so it's a little tough to analyze that. ;-)

Like @ConcernedOfTunbridgeWells pointed out in his comment, though, the real reason you should get off SQL Server 2000 is that it's out of support.