It’s probably a single dev that made the decision, then moves onto something else. They (probably?) don’t have the ability to just raise a recurring PO etc to easily pay you and don’t care enough to worth through the paperwork.
If you had a paid licencing model they may have done it, or just found another lib/ wrote their own.
If you have lots of tabs up top you can’t see the names, vertical tabs lets you see more/all of the tab titles.
I have been using the tree style tabs add-on for years, it’s amazing for a tab hoarder.