Nothing is preventing developers to sell their apps directly or on 3rd party stores. Sure being able to sell apps on the Jolla store is great, but it is not the only way.
Many of you coming from Android or iOS with the walled garden culture and a centralized store controlled by the OS vendor, might miss the open ecosystem that an open platform provides.
Such as Linux or even Windows. The lack of paid apps on most Linux repositories, or the lack of a centralized App Store in windows for years didn’t prevent developers building high quality apps and selling their software.
As @fridlmue pointed, the main issue is the lack a of diverse and big user base willing to pay for basic apps.
Home users on mobile platforms usually pay small amounts in micro-transactions and often under influence of aggressive marketing and behavioral ads.
The user base in SailfishOS, I understand it’s mostly developer, or tech enthousiastes, and many aren’t looking for a clone of Candy Crush, or to have a free app filled with ads and tracking, or pay 1€ or 2€ for a simple calculator or weight tracker. Maybe in the general public some are willing, but I doubt the user base here would do so sufficiently to make a developer rich.
The closest demographic of SailfishOS users, from what I see in the forum, are probably Linux users. Few software get sold in Linux, and they’re mostly for business or advanced users, or some games on steam. Donations seems to work better.