Could a million reasons,
But to know more try answering these questions:
If PHP in safe_mode? (hopefully not although its ok to have it)
Whats the PHP memory_limit? (Needs to be more than 8Mb)
Does PHP allow uploads? (Needs to be yep)
Whats the maximum upload size in PHP? (Something nice)
Whats the LimitRequestBody set to in httpd.conf? (Redhat sets this to 512k)
Do you have openbasedir on and a suitable temp folder?