My solution in this area has always been to make developments financially lucrative where possible through usage of the product being developed.
For me, the best way to attract more app packagers to Cloudron is for Cloudron to focus on making the platform a profitable addition to any business.
That probably means business monetisation or cost-saving apps being the higher priority, and entertainment apps more of a hobby.
It's probably why Wordpress gets more attention, as it's hosting is a common income stream for designers and developers.
Research and community assistance karma seems to be another good way to help, as the more the power users are here to answer community questions and use their eyes and online time to help with research and communications, the more time the app packagers have to focus on that.
Lastly, spread the word on reviews websites. I've watched the community grow in my brief time here, and apps packages from that thanks to that growth in interest and assistance with feedback.