As a professional-level baritone who has sung tenor parts quite a lot, there is a shortage of every low voice type (directors are often conflicted when I make the offer to sing tenor). People who can produce a chorally-acceptable A or Bb are in the shortest supply, though. It's getting worse as the amateur singing circuit gets smaller and the gender ratio gets more skewed.

Amateur-level choirs tend to have a lot more basses than tenors because it is easier to sing bass without effort spent on vocal training.

Tenor parts are more difficult, technically speaking, and voices capable of the tenor range are rarer. So any given man joining a choir can more likely manage the bass range, and if they can, they can almost certainly manage the bass parts.

FTA:

> When men do join singing groups, they often avoid the tenor section. The tenor voice is “a cultivated sound”, says John Potter, author of a book on the subject. A man with no vocal training is more likely to have the range of a baritone (a high bass). It does not help that the tenor voice is associated with operatic stars such as Luciano Pavarotti, who could powerfully sing high notes that no amateur can easily reach. And the tenor line in classical choral music can be difficult, with many unexpected notes and alarming leaps.

As the article states, its usually 2:1 women to men so there is also a kind of shortage of basses, its just not as severe.

I know many women who admit they "fall in love" anytime they hear a low bass. They might marry a tenor and never cheat on them, but every time their hear a low bass their heart flutters. Men know/see this and so tenors become less interested since their higher voices don't get the women (there are plenty of other ways they have).

I don't know how much this is a factor, but...