Asta's magic was unique. There was no one else who could cancel out another person's magic, so why didn't the other captains see the possibilities of that strong power?
There were many reasons. The biggest one would be the Nobility bias a majority of the captains had. Asta had also failed every task EXCEPT the final fight with him and Sekke, so his failures from those would also overshadow his one success.
Another major reason is that while we know now that he explicitly cancels magic with his swords, from just the one fight with Sekke that the Captains saw at the time, it looked like he simply overpowered Sekke with physical prowess alone. When he started his attack against Sekke, this is what was said:
Vangeance: He's fast! Is he using some kind of high-speed movement magic?
Yami: No, he's just physically powerful!
In the end, it was Asta's determination despite his shortcomings that won Yami over while everyone else only saw his shortcomings, and magic-cancellation wasn't on their radars at all.