shaddow, what they 'have against bryce' is simply that there is a mass amount of things in the library that look as if the program has done all the work, and they all look the same.. funky, abstract glassy warped and twisted shapes that really don't resemble anything more than crystal vomit. after a time, the admins are tired of looking at it over and over again.
they prefer to see work that, if done in bryce or 3d studios max or other 'autoted rendering' programs like terregen, is origional, unique work that doesn't look so basic or like it came out of a tutorial- with someone drawing random objects and applying automated effects.. work that you have to say 'wait.. what did he use to make that?' is held in high regard.
this may or may not sound like something you have submitted.. but that is the overall reasoning. they understand that all artists came from somewhere and i'm sure all bryce users started with the messy abstract shapes-but the library simply doesn't need any more of those. this reasoning is not limited to bryce, it goes for photoshop, psp, and all others. i've even had photography rejected.
keep trying.. i'm sure you will soon submit something that the admins think is unique and different, and you'll get in there.