DM plans to use a fumble chart and he’ll be damned if any players are getting out of a nat 1 easy by playing halfling

I mean, my best guess? Like chaotic neutral, players who “must” play halflings do it because they want to play to type… a very, very annoying type that doesn’t make for a fun time at the table… and saying this weeds them out.

Darksun during the Cleansing Wars?

The entire party showed up all using Zee Bashew’s lucky halfling diviner build and called themselves the Fellowship of the Rng

Boring possibility: the DM thinks halflings and gnomes are largely redundant, and picked one to ban.

Or the one shot involves outsiders getting involved with a halfling community in trouble!