Here's a hint as to why to keep closed source...so no one knows what sneaky things the developers are doing.
https://www.reddit.com/r/burstcoin/comments/c4v1yi/important_boom_coin_scam/Crypto is about trustless systems right? Closed source does not follow this. It is just greed. Do you want greedy developers for a coin that you spend time, money, and energy on?
BHD also says they will be having open source "soon". More like "never".
Understand your concerns. When we make sure there are no fatal security issues, we will open source at the first time. After all, we want to attract developers from the POC community to participate in this project.