It was awesome, super popular and could have been WordPress today.
They raised funding. Possibly under VC pressure, they made the license terms more closed source and restrictive.
Meanwhile, Matt Mullenweg had forked b2 and created WordPress under GPL.
When MovableType went closed source/more restrictive, their users fled to WordPress. The rest is pretty much history.
So what this historical context helps us understand is that firstly, WordPress/Automattic is very careful about ever going closed source with anything because they learned their lessons thanks to MovableType and what happened to them.
So WP has chosen to be open source, stay open source and defend open source because history has shown that is what the market wants and if you don't do that, they'll divorce you.
Living in that world as a multi-million (billion?) dollar company is a tough gig because, as Matt himself did with B2, anyone can fork your code and run with it.
But the one thing that WP/Automattic can count on is the GPL. If someone does fork the code, they are forever bound by the same competitive constraints that Automattic/WP is bound by. In other words: someone else can fork their code and so on in perpetuity.
So as long as WP enforces the GPL over WordPress, they will never have a competitor stronger than they are who can develop proprietary IP off their platform and gain a competitive advantage. The GPL ensures that anything created out of WordPress is always available to all competitors. And the competitor with the greatest head-start wins and that's WordPress.
This is really the basis for how multi-billion dollar companies are built out of open source. They count on the GPL to ensure everything they create never becomes a proprietary competitive advantage to a competitor that isn't also available to them.
So Matt M has a strategic imperative to always enforce the GPL on everything open source they do, or else they're building the foundation of the company that will kill them.