A rough rule of thumb is that code which solves a problem that exists in one or only very few projects is not convenient for inclusion into PEAR because most people will not benefit from it.
However, this does not mean that "niche software" cannot be included in PEAR. In the Math section we have a number of elaborate packages that solve different kinds of algebraic problems. While those problems do not occur in average applications, the packages are very abstract and scientific projects benefit a lot.
Apart from code that does not fit into PEAR because of a lack of abstractness, there is another type of code that currently cannot be contributed to PEAR: Full web applications. We do however accept developer tool application like phpDocumentor but not end user applications.
Despite the fact that the "A" in PEAR stands for "Application" there is a simple reason why applications are not yet supported: Until now nobody has gotten around to make the infrastructure like the PEAR installer capable of handling packages that provide complete web applications.
When the situation changes, this site will be updated.