Cryengine 3 costs something like $1.2m to license. COBRA is the in-house engine, so is free. Also, the devs are all familiar with working with their COBRA tools, so there's no learning curve. RSI have hired specialists to train people in using Cryengine.
It would be a big expense, and not really worthwhile, to use anything other than the in-house engine. When DB mentioned the new investors, he specifically mentioned 'tech and tools', so I suspect Frontier will be doing some work on updating COBRA to make sure the graphics part is up-to-date, and things of that nature.