Chromecast is the most popular media streaming device currently on the market yet many apps (specially on desktop) don’t fully support the ability to cast a movie you are watching to your Chromecast device.
VLC is one of the most popular video players and according to the VLC changelog we can expect a new feature in VLC 3.0.0 that will add the ability to directly stream your content to your Chromecast. The current stable desktop release of VLC is 2.1.5 and 2.2.0 is expected to be released in the next couple of weeks. Some of the new features in VLC 3.3.0 (source)
It is yet unknown when VLC 3.0 will become available, for now it seems like you will just have to do with the built in casting capabilities of Popcorn Time