They haven't done it since the rewrite. And it's disingenuous to state it as flat version numbers without context. 12.x was the old Opera-specific engine. Now, they're running on Chromium/Blink. Since the switch at version 15 (skipping 13 and 14), they've got Mac and Windows out (since they're the biggest). They're working on Linux behind the scenes but it's not ready yet. And they're now doing fast releases, so basically a new version every month or so. So, it's not like the Linux version is forever out of date. They just switched to the new engine in the last few months and haven't done the Linux build yet. That's all.
As a bit of history, in case you're interested: Opera releases often used to be staggered, with Windows out first (since, as you say, they're the biggest), MacOS following, and Linux several weeks behind. Around Opera 10ish (I think, can't remember the exact one), they made a thing of changing that, promising that MacOS and Linux releases would from then on be simultaneous with Windows, as equal-first-class platforms. And with the switch to blink, they've quietly dropped that. Obviously I see the economic logic of releasing for the most popular platforms first, but that doesn't much blunt the emotional disappointment (from me and other Opera for Linux users) of seeing them drop their commitment.
(On a more practical note, for why I switched to FF - the latest Presto may not be much more than a year old, but you'd be surprised how fast the web changes when you're using an unmaintained rendering engine. New website functionality often doesn't work - html5 file uploading on most sites is broken, can't get the new google maps, g+ and fb are slow, etc. Web devs don't test on presto now that opera for windows/mac doesn't use it, and opera have stopped packaging website shims).
10.50 was Windows-only too (the day after the brower ballot screen went live, 2 March 2010); Mac/Linux not catching up till 10.60 (1 July 2010).
(Note also that there had been large delays in Presto reaching Desktop before — most notably with Opera 10 (2.2.115) and 10.10 (identical but with Unite enabled), Mobile shipped a beta of Mobile 10 with Presto 2.3.something before 10.10's release, and a second beta with 2.4.15 shortly after. Note from late 2.3 Presto releases, the final digit is the most significant one.)
