Why would a scholar conceal beneficial knowledge?
Among the characters of those on whom Allah is angry is to conceal beneficial knowledge and be stingy in spending from this knowledge. Thus, the word “Bukhl” or stinginess in the Quran refers to both stinginess from spending money as well as knowledge
see verse 36–37 of Surah Nisa (4), verse 187 surah aal-Imran, verse 159, 160 of surah al-Baqarah, also 174 and 76 of this same surah.
Many of the above verse describe this characteristics as a characteristics of the Jews. They conceal knowledge: 1) out of stinginess (Nisa 37), 2) or to enjoy some worldly benefits (Baqarah 174), 3) or spreading the knowledge would give their opponent an excuse (Baqarah 76). One of these three motives is consistent in any one who conceals a beneficial knowledge.
When a scholar conceals beneficial knowledge, he could be doing this: a) out of stinginess and envy, so that his source of power is not shared with others, or b) revealing this knowledge would cause him lose job or social ranks, or c) he could have supported a view or opinion deliberately which contradicts the knowledge that he conceals so that others can not catch him.