URSA Mini 4,6K prores and BMCC 2,5K prores, why not ?

When we ask the question, the answer is always "because prores standard is 1080p, 2K, UHD or 4K". So why Alexa Mini now have 2,8K prores? Can't we really have 4,6k prores or 2,5k prores ?

Who knows. One of the things that comes to mind is a cost thing because it's not a standard format and Apple has to certify it. Arri probably paid a good amount of money for that. Your BMCC and Ursa Mini are $2000-$5000, not $70,000+.

It could be any number of things.

Apple does not have to certify the ProRes resolution that a device can encode. You can encode ProRes in any resolution you want in plenty of software packages that are available, including DaVinci Resolve.

We have a full length discussion about the 2.5k ProRes topic over here:
viewtopic.php?f=2&t=33014