Mostly because it is closed source, and also the odd behind-the-scenes drama of Resilio's spin out from BitTorrent (the company, not the tech). (The BitTorrent company of today is a strange cryptocurrency/NFT zombie of the tech company it originally was.) To my understanding that drama and corporate spin out in part even influenced the creation of Syncthing as an alternative.
From my view, Resilio is still easier to use with better apps than Syncthing, and in theory their corporate business model seems sustainable (more so than the previous parent company) and can provide useful corporate support when such needs occur. But there's still lingering doubts after all this time that they will continue to do the right things, support the software well, and it is closed source so there's not a lot of community support options if the company's business model pivots in any accidentally similar way to the events that lead to Resilio existing in the first place.
From my view, Resilio is still easier to use with better apps than Syncthing, and in theory their corporate business model seems sustainable (more so than the previous parent company) and can provide useful corporate support when such needs occur. But there's still lingering doubts after all this time that they will continue to do the right things, support the software well, and it is closed source so there's not a lot of community support options if the company's business model pivots in any accidentally similar way to the events that lead to Resilio existing in the first place.