... Opera Turbo, especially for those who have a consumer connection, was an essential function, one of those that should never be touched, so I don't understand the logic behind this decision.
I'm not an Opera employee, but my best guess is it was a business cost decision, perhaps accelerated by the recent Opera ownership change. Turbo was not a strictly browser-code feature as much as it was a proxy-like filter residing on servers operated by Opera (which compressed the data coming back from a visited website - especially images - thereby reducing the data package size going to the user). So two cost-reduction possibilities present themselves: either it was becoming too expensive to maintain the Turbo browser code in light of the continual chromium engine updates or/and it was becoming too expensive (or bandwidth-competitive) to maintain the proxy-like server network. My personal guess is the latter to be more likely contributor.