I've always liked Opera because it worked well even on older hardware, it didn't use a lot of resources and it included many useful features. Unfortunately, Opera's desktop market share has always been very small and many important sites ignored Opera. Many Google services displayed warnings like "unsupported browser" or had features that didn't work in Opera.
Opera Mini became popular, especially for feature phones, and Opera's focus started to shift towards mobile devices. Now Opera tries to focus on smartphone users and that's the main reason why it gave up on its own rendering engine and chose WebKit/Blink/Chromium. The desktop browser became harder to maintain and it only had 50 million users, while Opera had 250 million mobile users.
Opera's desktop browser is now based on Chromium and lacks most of the features I've mentioned. Here's Opera's explanation:
When we released our first browser in 1996, most web users were people who weren't afraid to tinker, and who liked lots of options and configurability. Fast-forward 17 years, and the Web is everywhere. Speedy browsing and sites working properly is the most important thing to many, many people.
That leaves us with the riddle that every software developer faces at some point: how best to make a UI simple enough to be intuitive for a consumer who wants a solid, fast browser that just works, and yet is customizable and extensible so that power users can add the features they want?
The answer is to build a strong, extensible foundation on which to innovate. Opera 15 is a fresh start, to which we will continue to add features.
While I'll miss the old Opera, I understand that the desktop browser is no longer a priority for Opera and building a Chromium-based browser is much easier. It's also an opportunity to start with a barebone browser and only add the features that are necessary, not everything but the kitchen sink. Chrome's comic book from 2008 still makes sense today.