"It is simply the fact that LINUX is much older than Be-OS"
I don't think Linux is any older, and certainly not "much". Be Inc. was founded in 1990, and BeOS claims to date back to 1991. It's old enough to have been written for the ill-fated Hobbit CPU architecture and had to be ported to PowerPC and then Intel.
BeOS was trying to be more "forward looking". So while Linux soon needed a traditional Unix "big lock" multi-processor conversion, BeOS was designed for symmetric multi-processor systems from the outset, and instead of needing _LARGEFILE_SOURCE BeOS always defined off_t as a 64-bit value too. However more often than not their crystal ball was cloudy. They wrote for the ill-fated Hobbit CPU, they assumed Internet connectivity was relatively unimportant, and they relied heavily on C++ before it had even been standardised, let alone settled on a maintainable ABI. As a result of that last misstep Haiku must rely upon GCC 2.x because that's the last compiler which implements the C++ ABI the original BeOS R5 is wedded to.