Because Mesa got a little "loose" with the naming conventions of the Rectifier series. The original Dual Rectifier was named so because it had TWO options for power rectification: Tube, or SS. The Triple Rectifier was named because it had THREE sets of rectifier tubes, and the single was named because it had only ONE option for rectification: SS.
I think.
The truth is, the Single Rectifier has a) a SINGLE set of power tubes, or b) a SINGLE option for rectification, but it doesn't matter. It's a 50-watt amp in the Rectifier series. It shares the majority of its preamp setup with the bigger amps, and has a similar tone and feel. It's a Recto, be in a Solo or a 'Verb.