Tabata said it was a joint decision with Nomura making the initial idea to remove Stella.
However, there are a lot of things that we do not know. First is, was the female character from the 2013 XV reveal trailer even Stella? That may already been Luna at that stage.
The character design change drastically in the 2013 rendition. Perhaps the switch to Luna was well underway by this point. You could argue and say that was Stella and the 2014/15 rendition was Luna, however, the current Luna looks practically identical to Steluna in 2013 and that can be proved.
This is a comparison of the various different renditions of Luna, compared the final CG render. The first comparison in the top left compares 2016 Luna to 2013 Steluna and with some minor nuances such as hair, it looks pretty much identical.
So what does this suggest? Perhaps Stella was in the process of a role change prior 2013 reveal, from being an semi antagonist to a full on side supporting character with the changes in story happening at the time. However, the character became completely different by that time and perhaps it was decided to remove Stella then. In turn this would make sense as Nomura was still heading the project, or at least still in the team at this time.