Maybe there aren't that many OGs who still have part of their stash on such a wallet. Luke is quirky and maybe sentimental about it.

Or he encrypted his wallet using his PGP private key which was stolen as alluded to elsewhere.

I mean the more I think about it, the less likely this is the case, since Luke is a public person who has a high probability of being targeted. 

BITC0IN

The tweet doesn't imply that the thief was targeting Luke specifically. Just blanket targeting all private keys generated with a bad random number generator.