Why do studios publish movies on multiple BD discs?

[smcmanus]

Hi,

I'm new to this forum, and I was just gonna throw this question out. I'm not sure if it's been asked before.

I was wondering why studios choose to manufacture Blu-Ray movies using multiple discs, mainly to include Bonus Features. If Blu-Ray discs can hold significantly more material, why not include everything on one disc and save money and materials?

Thanks!