They have an official explanation on this:
https://github.com/zcoinofficial/zcoin/wiki/FUDWhen Zerovert was launched in November 2014, it was primarily an experiment and we didn't have any investor backing. We still wanted to protect our work and thus our only choice at that time was to make Zerovert closed source so that it could not be copied. However we quickly realized that in the cryptocurrency world, closed source software made it almost impossible for our project to be vetted and hence taken seriously so we were in a quandary.
Zerovert was originally intended to be part of Vertcoin's ecosystem however Vertcoin's community were not keen on it. Zerovert's also suffered from very poor performance and had huge scalability issues and therefore the project went silent while we figured out how to solve many of the challenges we faced with Zerovert.
We subsequently managed to fix many scalability issues and these changes have been incorporated into Zcoin. Starting afresh with Zcoin allowed strong investor backing from the likes of Roger Ver and allowed us to dedicate ourselves fully to this project rather than a mere experiment. With investor backing we also could finally make our code open source while still protecting our efforts. These considerations were what prompted us to wipe the slate clean with Zcoin instead of continuing with Zerovert which was never intended to be a serious project.