Yeah, the Monochromes got started last year sometime. They're largely going up in areas where the signs need replacement or where there's construction going on. My guess is one of two reasons - either cost (green ink costs more?) or contrast (they do look better at night, I guess.)
As for Series B, Louisiana used to use other fonts as late as the mid 1990's, and they still use them on freeways that carry a state highway designation (I-510/LA 47 in New Orleans East, US 167/LA 28 near Alexandria as examples) or at freeway interchanges or offramps. (These are also the few times one will ever see a wide shield used for a state highway in La.) Sometimes they can even be seen as part of sign goofs as well...

...At least La. didn't decide to go to Plain White Squares/Circles...