They ship Manjaro ARM because the people behind PINE64 are manjaro fans and they think that Manjaro has a good business setup. It would have been much nicer to have PostmarketOS shipped stock.
Additionally, pine64 have already worked with them on the pinebook and pinebook pro as default distro.
Personally, I don't like Manjaro either, but it's easy enough to use a different distro and is probably good enough for most.