OP already got the answer, but Sonya is also more versatile which is something to keep in mind, and she’s also a bit of a keystone hero for her color.
Riposte is very situationally useful and generally doesn’t require a fully emblemed hero to maximize its effectiveness, and Cyprian is easily outclassed by any 5* tank, so he’s more likely to become obsolete. Having Cyprian emblemed increases the likelihood of him surviving long enough to fire on bad boards, but chances are good that if you have a bad board, you’re already on the ropes, and Cyprian is unlikely to get the most usage out of his special when that happens due to his neighbors dying. Also, the Paladin special increases his defense which does help with the survivability but not so much with the riposte.
Boril by comparison is a much better Riposter in general as his inherently high defense means he’s very likely to survive to fire, and as a Cleric the mana shield means he’s less likely to be affected by mana cuts or special blocking effects like Merlin.
So that said, Sonya is a bit of a keystone hero for blue because she’s the only blue dispeller for most people (the other being Frida who is a previous HOTM and is unlikely to see rotation anytime soon). This makes her incredibly important for several problematic red heroes: Ares, Boldtusk, Mitsuko (to a certain extent, Sonya will have to punch herself but clears the path for follow-up), Elena, etc. While blue can kinda dispel via overwriting effects, sometimes you just need the effect straight up gone, and there’s only one hero (for most) that can do that job in blue.
And yes while other colors have dispellers, it’s better to have a strong-color hero who can do the job than a neutral (or gasp weak color like Green vs. Red) to do the job if possible.