Perhaps the low adoption of users and low usage of available storage is the reason for SC dropping. I have not tested Sia from a end user perspective, but if it is not super easy to buy storage as an end user, they will not attract private users who I think is a great market.
It's not overly easy and not ready yet for average Joe use. However that is not the goal. The aim is to be a back-end for 3rd party services.
It's more important to implement certain features first. believe me. you don't want to release a product for mainstream use before it's ready. first impressions count. that's why they're not promoting it as of yet.