When the show got renewed at the end of last year, the producers did say series 2 wouldn't be ready until 2017.
I would imagine filming will be a lengthy process with 8 hour long episodes so I doubt it will wrap much before the end of this year, that will be followed by post production so I would imagine the earliest we could see series 2 is Apr/May next year.
I think the main issue is, is that they wouldn't have been able to put a filming schedule together until early this year. When the show finished airing late last year there was a period where no one was sure if it was going to get a second series and then when the green light came BBC America pulled out and the BBC had to persuade Netflix to come on board which would have caused further delays.
Due to all the uncertainties the returning actors may well have been working on other projects, Dreyman has been involved in a couple of projects and Ian Hart is always a very busy actor.
My best guess is Series 2 should be on screen by the middle of next year so that's only 18 months in between series and that's not uncommon for a BBC show.
There is a lot to film, but one would think it's just what they do and they would be less beholden to seasons than the viewer. At least no more than most shows. Shame.
Ratings dropped on The Musketeers which I loved BTW so the BBC canned it.
Shame too - it was a great show with everything I love, action, romance, beauty, and it was in the past, although, with a lot of dramatic license added in.
Regards the Musketeers I think the BBC made it bit of a mess with it's broadcasting.
The first series went out at 9 o'clock on a Sunday nights and that seemed to work. The second series was moved to Friday's and because there were a number of sporting events on Friday nights at that time of year, 3 or 4 episodes got delayed which effected audience continuity.
The final series was just a mess as they had to fit it in around their coverage of the Olympics and Euro 2016. Some episodes were shown on Monday's and some on Saturday's and it was very difficult to keep track.
However it may not have just been down to schedule and ratings. The cast were all given contracts at the start for 3 series. For the series to have continued beyond a 3rd series they would have had to have resigned everyone which might have proved quite difficult. I think both Luke Pasquelino and Santiago Cabrera were keen to move on. They could have of course introduced new characters to replace them but it wouldn't have been the same.