The Shaagas Aryeh's Sefer

Rav Neta Freund related that the reason the sefer Shaagas Aryeh deals so much with Maseches Berachos was simply that the Shaagas Aryeh was so poor that he could not afford to buy a Shas, and he did not own many volumes of Gemara at all.

A wealthy merchant woman who recognized the value of Torah study owned a large, beautiful Shas. She used to lend out one volume at a time to various rabbanim and talmidei chachamim as they needed. To the Shaagas Aryeh she lent the volume of Maseches Berachos. A week later the Gaon tried to return it and exchange it for another masechta. She answered, "Oh no, you cannot fool or cheat me so quickly. No one can finish learning a masechta so fast, no way."

And so the Shaagas Aryeh was forced to keep Maseches Berachos for a long time, and much of his sefer was therefore written on topics found in it.