Doesn't make much sense to keep the address in uppercase as Bech32 and Base58 are designed to prevent human error in the first place. Maybe I will open a pull request for this.
I think it's because it's stated in the proposal.
BIP-0173 strongly suggests to encode Bech32 address as uppercase in QR Codes:
github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#bech32(it is a bit irony that bitcoin: part is lower case, whilst address isn't)
Setting it to uppercase will further increase the incompatibility of the QR Code.
There's a discussion about that in Bitcoin Core's GUI repo GitHub:
https://github.com/bitcoin-core/gui/issues/145