Recently Valve has released an update to their CS:GO hosting policy that prevents server hosts from hosting any servers with custom models. This was released with good intentions to prevent server hosts from selling items for their server only, but in the process Valve destroyed the modding community, all to make a tiny bit more money.
Valve even advertised a working zombie mod while the game was still in beta, but that seems to have been put on hold. We do hope that Valve will fix this one day, but it is very doubtful, as they did the same thing with Team Fortress 2, and haven’t fixed that for a long time.
For now to get your zombie fix you can always play some No More Room in Hell!