That's because they've all been sold. There were $10-15 SRO for tonight and tomorrow's game available last week.
edit: Still $20 Bud SRO for tomorrow and all kinds of SRO for this weekend.
Bud SRO is available for all games, from what I can tell. And right field roof deck are also available for some games.
But for tomorrow's game, I could not find even a single (non Bud deck) SRO ticket on redsox.com, stubhub, ace, or seatgeek, despite plenty of bleachers, grandstands, and other seats. It seems hard to believe that those crappy GA SRO tickets where you stand behind the line at the back of the grandstands are so in-demand that nobody is willing to part with a single one at any price, for a Wednesday game against the Orioles in April. And same with a weekday series against the Blue Jays in July: plenty of seats from redsox, stubhub, ace, etc, but no SRO (except bud). That was what made me think that maybe they hold SRO tickets until they hit a certain sellout ratio, or something.
And when I think about it, it actually makes sense. If there are plenty of empty seats, why would the Red Sox sell tickets to stand behind those seats, when they know those people are going to sit down in one of the empties? Alternately, that could be an explanation for why people are willing to sell grandstand seats but not SRO tickets. But I would be surprised if the market is that rational in one direction and not the other. The simpler explanation is that Bud SRO tickets, which are restricted to one area, and which sell for about the same as bleacher seats, go on sale first. Then they release other SRO tix as sections fill up, or something like that.