Absolutely nothing to do with "learning new things". There's really nothing "new" to learn about QB online.
It's the fact that many businesses utilize features that the desktop version of Quickbooks HAS...and the online version of Quickbooks does NOT have. Such as..payroll services, or certain reports, or...(a popular one)..integration with 3rd party apps they run their business on. The online version of QBs is "neutered"...certain features removed. Some businesses have run on, and rely on, those features.
As for accountants..yeah, think about how they run the accounting version, and how easy the import/export/import/export feature is with their clients. They run versions of QBs that can open QB company files made by many diffy versions of Quickbooks. That's mostly gone with online. So...they're stuck without a tool.