Contributors here are all end-users, just like you. Other than information published by Apple, nobody here has any insight into Apple’s product design decisions or future software development roadmap.
There are some features and native Apps that are only available for iOS (iPhone) or iPadOS (iPad) - the Journal App being just one of the Apps that are still exclusive to iPhone. As for why Apple choose to make some Apps available to one or other platform are known only to Apple.
Apple invite submission of comments and feature requests via its Product Feedback portal. If you would like to see a specific feature or App in a future version of iPadOS, this would be the most appropriate channel through which to make a request:
Feedback - iPad - Apple