Why?? Please start this feature again. It was a nice feature.
The official "why" would have to be answered by Opera itself. However, the way Turbo worked was that it acted as a 'proxy' between the user and the selected website, and it used algorithms to compress the website's graphics/image (primarily) data stream flowing to the user by employing that proxy server. Since Opera had to provide/lease their proxy servers, it incurred the expenses of maintaining the Proxy server system and software - yet the Opera browser is free. Only Opera would know if the costs of operating the proxy servers was/might still be worth the benefit level to a sufficiently large enough portion of its users from making the service available.