If eAccelerator uses an opcode cache for PHP requests, then why don't all companies employ this on their production servers? Is there a reason why Apache/PHP/mod_php doesn't bundle it with their package?
Not all packages/scripts play nice with eAccelerator. And eAccelerator will never make it into PHP, because APC -- Advanced PHP Cache -- is set to be included in PHP 5.4, and is generally considered better than eAccelerator.
Also, not all web servers use PHP as an Apache module (mod_php); in shared web hosting, it is quite common to run PHP on a per-user basis with suphp or fastcgid. eAccelerator doesn't work well with fastcgid, much less so suphp.