I have discussed SRS with a lot of people, and most feel (incorrectly) there is too much overhead in using the system compared to the gains. Even for many people who try it, it feels wrong somehow.
Saving a minute now is more gratifying than the promise of saving hundreds of minutes over the course of your life. Consider compound interest. We all know the future gains we will receive by investing money will be greater than the current value, but we choose the immediate gratification of spending now.
Also, even though the technology is getting easier, effectively structuring learning will always be difficult. If you just want to memorize a bunch of items--that's easy. But if you want to break something down that is large and complicated, and be able to use your knowledge creatively--that's hard. I have been using SR to learn jazz improvisation, and I had to read dozens of books to figure out how to structure my learning. I had to become an expert on the pedagogy before I could even begin SR.
Finally, SR alone is not optimal for creating fluency. You cannot really learn to converse with native speakers by memorizing every word and grammar rule in their language. You have to use the language for a purpose, and communicate with native speakers. SRS needs to be part of a balanced learning program.
I think we may be reaching a tipping point soon. The technology is getting easier. The benefits of SRS are becoming more widely know. I think that you will start seeing content experts designing SRS programs that are balanced with other activities. There was a good article at Salon.com about how the government is investing a lot in language education and research, to get people up to speed with Middle Eastern languages. The three essential elements are: SR; use language for a purpose; interact with native speakers.