It was politically contentious for SMC to be closed source, due the connection with the Sage open source math software project (see  for a big thread attacking SMC for being closed, and  for a rebuttal). SMC was closed source for years, primarily because my university commercialization office wouldn't work with me unless SMC was closed source.
In December 2014, I discovered that SMC had received some development support from NSF grants that had explicit requirements that all code written under those grants be released under the GPL (very few NSF programs have open source requirements, but the SI2 NSF program recently added them ). As soon as I realized this, I complied with the conditions of those grants and open sourced everything. This has had no real impact on SMC development. This didn't surprise me -- many of the other responses in this thread express the same expectation. I wasn't surprised because I started the Sage math software project back in 2005, and grew it from 0 to hundreds of developers (over many years), so my expectations for what it would take to get useful contributions to SMC were realistic -- massive, insanely hard work over many years, many workshops, writing lots of documentation, recruiting developers, arguing for why my project is better to contribute to than the many competitors, etc. Building an open source project is like building a company -- you have to recruit and train every new "hire" (contributor), build up process, sell a vision, etc.
My situation is unusual, since I am balancing simultaneously being both a (tenured) mathematics professor at a university and founding a company. Technically the university can assert ownership over anything I write using university resources (see ). However, due to the viral nature of the GPL, it doesn't matter who owns what I write, since I (and everybody) can still use it. I didn't want to use the GPL for SageMathCloud (I much prefer the BSD license), but technically I have no choice.