As stated in the title the reading for 文言 can be もんごん or ぶんげん. I know that もんごん is almost always used in speech, but why do two readings exist and why is もんごん so much more common? Also, are their any specific circumstances when ぶんげん is preferred?
もんごん is the 呉音 reading, and ぶんげん is the 漢音 reading. Although kanji compounds (熟語) can in principle have both readings (in addition to any number of customary ones), it seems it is more common to have only one.