When the brain has changed.

As for Mac OS, System 7 was a 32-bit rewrite from Pascal to C++ that introduced virtual memory, and improved the handling of color graphics, memory addressing, networking, and co-operative multitasking. Also during this time, the Macintosh began to shed the "Snow White" design language, along with the expensive consulting fees they were paying to Frogdesign, in favor of bringing the work in-house by establishing the Apple Industrial Design Group. They became responsible for to crafting a new look to go with the new operating system and all other Apple products.
Despite these technical and commercial successes, Microsoft and Intel began to rapidly lower Apple's market share with the Windows 95 operating system and Pentium processors respectively. These significantly enhanced the multimedia capability and performance of IBM PC compatible computers, and brought Windows still closer to the Mac GUI. Furthermore, Apple had created too many similar models that confused potential buyers.
 http://en.wikipedia.org/wiki/Macintosh#1990_to_1998:_Growth_and_decline