This is by design. iPhoto is template driven and the templates are fixed. This makes it very easy for inexperienced users to make an attractive slideshow fast. If you've out grown those templates then you've outgrown the iPhoto slideshow feature and i's time to look for a more capable app.
Alternatives to iPhoto's slideshow include:
iMovie, on every Mac sold, as is iDVD.
Others, in order of price:
PhotoPresenter $29
PhotoToMovie $49.95
PulpMotion $129
FotoMagico $29 (Home version) ($149 Pro version, which includes PhotoPresenter)
Final Cut Express $199
It's difficult to compare these apps. They have differences in capability - some are driven off templates. some aren't. Some have a wider variety of transitions. Others will have excellent audio controls. It's worth checking them out to see what meets your needs.
However, there is no doubt that Final Cut Express is the most capable app of them all. You get what you pay for.
Regards
TD