Like everyone else, they have to decide what they support – their core product pretty heavily depends on known file system semantics – and they gave advanced notice specifically so you didn’t have to “drop everything” if you for some reason cannot have a partition using a supported file system.
Put another way, do you think the combined users of file systems which aren’t supported ext4, xfs, btrfs, or zfs are willing to pay more or would quietly accept the possibility of data loss? I doubt the former is true and have absolute certainty that if there was a bug using an unsupported file system that would result in angry, hyperbolic blog posts saying Dropbox is unsafe and will lose your data.
Oh are those all supported now? They've clearly backtracked massively. When I bounced they were insisting on ext4 only. Glad I didn't bother rebuild all my machines to ext4 only for them to change their damn minds. They gave us less than three months notice. I'm not playing chicken for three months paying their professional tier in the hopes they change their mind. Freaking circus.
Put another way, do you think the combined users of file systems which aren’t supported ext4, xfs, btrfs, or zfs are willing to pay more or would quietly accept the possibility of data loss? I doubt the former is true and have absolute certainty that if there was a bug using an unsupported file system that would result in angry, hyperbolic blog posts saying Dropbox is unsafe and will lose your data.