Because it never really enjoyed any sort of actual push by Microsoft to get an app marketplace going on the system and their marketing quickly reverted to "look you can run real Windows apps on the Surface Pro". And now the platform is EOL.
Technically it had it together, it was just managed incredibly badly by Microsoft. People buying it and not realising it wasn't 'real' Windows is a failure in how the product was positioned and sold.
Is there a single report of one person buying RT thinking it was full windows?