First of all, this is not "Core's decision". Bech32 was not created by "Core" and has not yet been adopted and implemented by the other developers of Core. It is the proposal of a few people who work on Core but they are not and do not represent Core.
This is all explained in the BIP here:
https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#rationaleWhy use 'bc' as human-readable part and not 'btc'? 'bc' is shorter.