I'm playing on FTP SSS and over 20k hands now have lost just over $20. During this time I've made a few basic observations as to why I think this is the case:
1. There is a HEAP of people doing the SSS, this makes finding good tables quite challenging and if I'm not paying close attention to make up on each table I'm playing, it is quite common that I'll suddenly find I'm playing against 6 other players with $2 or less.
2. A LOT of people know how SSS works. If I get a playable hand and do the standard 4+1 BB raise, one of 2 things happens; it either gets folded and I pick up 15c of blinds (I'd say 75% of the time), or secondly I end up in a race with another SSS player with KK vs. AA or QQ vs. AK etc (probably 23% of the time)... honestly, only 2% of the time am I lucky enough to have a donk call me with cards that I'am well ahead of. Unfortunately this is not often enough to break even close to even due to blinds.
I do believe that maybe even only a year or two ago SSS would've been quite profitable, as your average poker player at micro stakes was rather lousy. These days a Very large percentage of players are well informed on concepts like starting hands, odds & equity, implied odds etc...