Be was one of the big players in the Microsoft antitrust lawsuit.
The part that didn't get pushed hard enough was control of the boot loader.
Microsoft stipulated that Windows OEM's couldn't install any third-party boot loader on OEM machines.
Be was making deals with big players such as Sony to dual boot Windows and BeOS so that users had the option, but MS put a stop to this. In order for Sony to not break either contract, they had to install both OS's, but make the user manually enable BeOS as an option. This was too cumbersome for normal users to really deal with, so it went basically unused by anyone other than those already dedicated to BeOS.
Combined with Steve Jobs returning to Apple and killing the Mac clone licenses, BeOS lost any real chance in both PPC and x86 markets.
The last attempt at staying afloat was in set top boxes (sound familiar?) but it was just too early for the market to support it.
They eventually ran out of cash and sold to Palm who didn't really do anything with it.
Before they went completely under, certain parts of the OS such as Tracker (think Workbench) were open sourced. This is partially why Haiku has gotten so far in such a short time compared to some other clone OS's such as AROS.