Why isn’t the split keyboard available of iPad Pro 12”?

You are certainly not the first to note that Split Keyboard is not available on the largest models of iPad, where many Users would perhaps argue that it is most useful.

Apple, for reasons unknown to us, have not included the Split Keyboard feature on any generation of iPad Pro 12.9. To my knowledge, Apple have never published their reasoning for this design decision. The iPad User Guide for iPadOS does declare that the feature is not available for some models of iPad Pro:

https://support.apple.com/en-gb/guide/ipad/ipad997da62f/ipados

“The split keyboard option is not available on iPad Pro 11-inch and iPad Pro 12.9-inch (3rd generation and later)”

If the feature is important to you, Apple invite constructive feedback and feature requests via the appropriate Product Feedback page:

https://www.apple.com/feedback/ipad.html