Because it was never open source.
Further, I'm pretty sure Lutz wasn't planning on keeping it free forever. If he were planning on making it free, he wouldn't have given it to a company, that sells products, he would've released the source.
The source which, even through itself is cryptic because it uses an obfuscator, and other build tools that consolidate multiple components into one. If he wanted it to be free forever, he would've released the source.