Are there specific threads you have in mind? Just because it has over 100 pages doesn't mean it's popular enough to pin. For example, the original Shuffle! thread has over 200 but it has very little activity these days.
Also, pinning topics is typically used for threads the FTV team wants to bring attention to, such as Forum Rules or FAQs in the troubleshooting section. So if too many topics are pinned, it may be harder to find those threads. And popular threads are usually at the top of a forum so they shouldn't be too hard to find.