Several possibilities- 1) very few people necessarily believe in radiant powers per say
2) it's a fundamental truth of human existence that we warp our perceptions to what we want to and/or expect to see. Lighteyes would neither expect nor want to see a darkeyes rebel as a being of power
3) everyone bar Dalinar seems to think Radiants are had
4) idiot ball for the sake of plot
5) several people ARE suspicious of Kaladin. Adolin for one pretty much did suspect it. Hoid KNOWS he is. Dalinar suspects it albeit a bit late. King T knows he is. The person who probably should have guessed but didn't was Shallan tbh, though she was probably thrown off when Kal couldn't heal himself.
6) finally for now, Kaladin deliberately didn't use his powers in a very obvious way- nothing overt except in front of his crew who are too loyal to blab. As for the fights, he already has a reputation as skilled from the tower rescue, and he DID beat a shardbearer before any active surgebinding powers manifested (so Amaram for example, who knows some arcane stuff is coming back in his own secret society, could believe his skill without attributing it to surgebinding). Adolin is also very skilled with a different weapon but nobody thinks he is a radiant.
Once it became known to most characters that Radiants were back, (and to some of those who knew they might be and anything about them) the dots would be easy to join, but before then the idea of Radiants returning would have seemed impossible, with their powers seemingly thought of as mythical.