Possibly to avoid conflicts with Nethack4[0], which was a fork of the Nethack 3.x series back when development was stalled. I think the guy behind it later joined the main Nethack dev team.

[0] http://nethack4.org/

There is a variant named NetHack 4, they might want to avoid confusion with it.

Nethack 3.11 for Workgroups was right there ;)