In addition to other reasons already given, commercial software may contain licensed code, libraries, assets, trademarks, and other IP that cannot legally be given away for free, or under an open source licence.
Sure, it may be possible to strip those things out, but that may leave the software broken or fundamentally changed, and it may be a significant amount of work to do, which am author or publisher is not likely to spend on abandoned software, especially if their free release would compete with any current products.