Ryan Schultz points us to Inara’s article SANSAR: Why LL Are [sic] Building Their Own Engine? It is a longish article and yet concisely answers the question providing several base reasons.
To condense her longish article to a few sentences… The primary reason the Lab chose to build an engine was User Generated Content (UGC). Other engines are NOT designed to handle the novice designers unoptimized content nor manage Intellectual Property (IP) rights. An added benefit is being able to use third-party tools, think Marvelous Designer.
A couple of things they are thinking about is Open Source. The Lab has become far more appreciative of open source over time. So, they are still planning on having some parts of SANSAR eventually go open source. Unfortunately, that will not be anytime soon.
The Lab is planning to support more VR headsets. OpenVR being one of them. I assume that includes supporting more controllers too.
A new aspect, new to us, is in the ability to run SANSAR on Android and iPhone/iPad type devices. The Lab has always said they want to do this. New in this iteration is they mention whether or not an experience will run on mobile devices is going to be up to the experience designer. So, some complex builds would be limited to desktops with greater render power, simplified builds could run on mobile devices too. So, they are NOT adopting an everything-runs-everywhere philosophy.
There are more details in Inara’s article.