Why isn't Vladimir Kozlov being used as a serious character?

WWE is using Vladimir Kozlov as a comedy character because he couldn't get over as a heel. Kozlov was in line for massive push, but when his match against Triple H and Edge at Survivor Series 2008 flopped, he was immediately de-pushed. WWE feels his look is marketable, but the reactions he was getting from the crowd were that he couldn't work, not that fans hated the character. The same thing happened with The Great Khali.