Because Kas'im doesn't have the feats to prove that he has the skills to pay the bills. Maul has outclassed famed swordsman such as Anoon Bondara and Qui-Gon Jinn. The former is actually very similar to Kas'im himself--A muscularly defined Twi'lek that has mastery of all 7 forms of the lightsaber.
Kas'im has his accolade of being the greatest swordsman in the Galaxy--But this is from Bane's perspective, and is probably untrue considering Raskta Lsu seems to outperform him. Still, his supposed perfection of every move and sequence of the 7 lightsaber forms is impressive, and he was PoD Bane's superior as a duelist. However, it's best we just be honest here--Stalemating "the" master of Soresu in a lightsaber duel (Per TCW) is quite more impressive than that. Not even mentioning the fact that Maul is a master of Teras Kasi as well as Lightsaber combat.
Force-Wise, his Force Defenses are pretty beastly, but he hasn't displayed offensive Force Prowess--Maul has (TK-ing an ETA-Class shuttle). Overall Maul is just more impressive. Kas'im gives him hell, though.