The development started in 2013, but it's, at this moment, not finished. Steve Gibson commented that it's almost complete and is just working on the documentation. So it's not that weird that it's not adopted in mainstream sites.

Just commenting on timing here, not the pros/cons of SQRL, which is a whole other discussion:

It's been "almost done" for a long time now. At Gibson's 2014 digicert presentation, he said he would be able to demo it in about a week. Nope, that initial demo didn't happen for another 6 months, IIRC. Shortly after that demo--we're talking June 2015--the Windows client was (checking transcript) "almost finished." Today is exactly TWO YEARS TO-THE-DAY later, and I don't think the client is yet complete.

I'm all for doing something right, and that's his stated reason for why it's taking so long, but it also means that based on past history, you should take his time estimates with a HUGE grain of salt. He's slow--it has been nearly 200 weeks of time where he claims he has been working on this more-or-less "non-stop" and that it has been his "focus."

I agree, I've been hearing that this is "done" for years! He's not even supposedly working on his main product "Spinrite" because he working on SQRL.

Steve is writing an official reference implementation for Windows. He is trying to get everything right and updates the spec with what he learns. I believe there are other unofficial implementations currently.

I'm a fan of it but I doubt it will gain traction just because it's a little unfriendly for businesses dealing with average Joes who can't be trusted to take full responsibility.