He's hiding it because he thinks that if some Captains find out that he has achieved Bankai, Ikkaku might be made to become Captain as three divisions are currently Captainless. He's very loyal to Kenpachi, his Captain of the 11th Division, and doesn't want to leave his division (he wants to die under Kenpachi's command), so he hides his Bankai, even if a few people know that he has achieved it. Not exactly loyal to Kenpachi.. Ikkaku doesn't want to leave because he has something that makes him wants to defeat and probably surpass Kenpachi.. Just like Renji is with Byakuya