Feels as if, hey made both Sam and Rdm with intention "to get new players hooked fast". One (Sam) with insane dps, not easy but not punished rotations/gameplay (punishing classes, frustrate new players the fastest), ..

the other (Rdm) very easy rotations/gameplay with very high burst and movement (+ thanx to utility gets in pfs well),

think SE felt Drk, Ast & Mch (roration/gameplay & "power " feeling-wise) didn't not get new players hooked that well ..., sure, there were a few new players, who still tried to stick to Drk, for what it represents, ... but at HW release, those 3 classes felt overall less attractive for newer players ..

probably a reason why Sam and Rdm can both be accessed in early areas, even ufo only at lv 50

maybe till SE feels, there are enough (new players) sticking to Sam/Rdm, they won't change much on them