As I mentioned in the thread about colored shirts, I have seen it brought down in seforim that there are inyanim to wear black or/white clothing. I imagine that it is for those reasons that the minihag became to wear black.
I see the opposite of Jothar. I see that although there has definitely been a great yeridah in the depth of our learning, there has been an incredible aliyah in the quantity. I’m sure that we haven’t seen such numbers of people learning on a regular basis since the times of the gemorah. It could be that since there are so many more “mini”-talmidei chachomim (as opposed to a few major-talmidei chachamim) the minhag spread more thoroughly to follow these inyanim that are brought in seforim.
Think about it. If 3 out of 10 people read about “A”, then they may be the only ones follow it. But if 7 out of 10 people read about “A” then you will likely find all 10 people doing it.