Well if you're only playing the one champ, that's most likely the issue if you already got your soraka chest for the season.
ARAM can be nice to get the occasional S rank on champs for roles besides the one(s) you play if the stars align just right and you become a god, but branching out for a little bit in normals for the sake of chests can be a worthwhile endeavor. (I'm a support main, so while I've gotten every possible chest so far, I'm almost out of even remotely viable 'supports' to get my chest for, and I'm cautiously optimistic whenever I get a non-support in aram now.)
Also, just to make sure you're aware, theres 0 randomness involved in getting mastery chests, it's just a matter of getting an S rank on a champ who hasn't earned one while you have a chest available. (Also, getting honoured doesn't actually boost key drop rates, just the rate at which you level up honour. Key fragments themselves drop based on number of games played these days, so if you play a lot, you'll get a bunch of frags [up to 12], but getting honoured 4 times per game or 0 times per game won't matter provided you're level 2+ honour already.)