Det beror på schemaläggning.
När du har två ledare som också presenteras i stora filmer som Hobbiten ... (tillsammans!) och Star Trek plus att ha författare och producenter som är involverade i en mängd olika projekt, är det svårt att få alla på samma ställe samtidigt för att göra en show ... speciellt om du vill behålla kvaliteten på showen på den nivå som folk är vana vid.
Från en artikel i Telegrafen :
[Moffat] confirmed he hoped to make further stories for the popular BBC ONE show but said it was a case of fitting in with everyone's diaries.
"Well, apparently Benedict and Martin are quite popular in the movies these days so it's quite difficult to schedule around them. And obviously Mark and I have our other commitments too, but it's just a matter of scheduling. We're all keen to continue."
Här är en annan artikel från Collider i januari 2014 som förklarar det ännu mer detaljerat ... men den grundläggande informationen är densamma.
Even though Sherlock hasn’t quite finished its Season 3 run on Masterpiece on PBS, fans of the popular series are already anxious to know when it will return with new episodes. Thanks to the increasing work schedules for show stars Benedict Cumberbatch and Martin Freeman, who have very busy acting careers, and Steven Moffat and Mark Gatiss, who write the episodes, the gap between seasons doesn’t seem as though it will shorten, anytime soon.
Are you working extra hard to shorten the gap between seasons, or do you just have to deal with scheduling?
MOFFAT: We deal with scheduling. I’ve also got to do Doctor Who. I’ve got no choice about that. That’s the day job. Everyone is a little bit busy. But also, it’s worked to make them wait. If we made Sherlock the ordinary way, and did a run of 6 or 12, it would have been over by now. It would have been done because Martin and Benedict would never have been able to find the time, after the first [season]. It would be done. This model of the TV series will happen again. Because we all love it so much, this could go on for a very, very long time. You’ll get to see an awful lot more of it.