Using Bitcoin keys is more difficult, you need to get people to use legacy addresses, as there's no message signing standard using segwit addresses. Awkward situation.
And what's the trouble keeping a legacy address?
Also it IS possible to sign a message with a segwit address.
Difficult, not impossible
1. There's no standard to sign messages using segwit. Bitcointalk software would need to implement different ways of verifying messages for different wallet software
2. People would have to figure out how to switch their wallet to generate legacy addresses, which is different in different wallet software
PGP is better for those config/standardisation reasons, but Bitcoin's cryptography is arguably more secure. Awkward situation.