For each letter in the aleph beis, there is at least one misechte in shas whose name starts with it — besides for every 6th letter (vuv, lamed and tzadi).
Did anyone ever hear any explanation of this?
See here that the letter (chart on the right) that the letter tzaddi - צ - has one of the lowest frequencies in the Hebrew alphabet. Only tet is lower. That is from anywhere in the word. A better frequency chart would be for the start of words.
In terms of vav, while it is frequent even in the beginning of words, this is only as a connective letter, meaning "And". How many words overall do you think honestly begin with a vav? And how many masechtot do you think would begin with the word "and" in the title?
Lamed is also frequently a connective, but admittedly also begins known shorashim. I wonder what the frequency is.
This might be posed as a statistics question. Given 63 masechtot, about how much coverage one would expect to have assuming equal probability for each letter? What about when we take into consideration the relative frequencies in Mishnaic Hebrew?
Alternatively, this is a question for the lamed vav tzadikim.