You're probably well aware, but for those of who aren't, he moved on to Vivaldi.
If that's the guy you are thinking of, he was no longer at Opera when they did the switchover.
He stepped down as CEO in 2010, left in 2011, and the switch was announced in 2013.
I speculate the cause is that the UI is written in HTML/Javascript, but I'm not sure. Some parts of it have to be tailored per platform, and be natively implemented, I'm sure.