The zombie reflexes answers already given are excellent, there are two additional reasons that might come into play as well though.

The first option is that zombies may be weaker or less co-ordinated than a living body, magic sustains them but they are still weaker and slower than they were in life. This is a decision you can make as to whether you want them to be stronger or weaker than in life, faster or slower, and also how much damage is needed to stop them moving.

The second option is that they may have specific weaknesses that human defenders know about (after all if zombies are common so are counter measures). Maybe a blow to a certain area kills them, or holy water burns them, or grave dirt prevents them passing, etc.

The zombie intelligence and reflexes answers already given are excellent, there are two additional reasons that might come into play as well though.

The first option is that zombies may be weaker or less co-ordinated than a living body, magic sustains them but they are still weaker and slower than they were in life. This is a decision you can make as to whether you want them to be stronger or weaker than in life, faster or slower, and also how much damage is needed to stop them moving.

The second option is that they may have specific weaknesses that human defenders know about (after all if zombies are common so are counter measures). Maybe a blow to a certain area kills them, or holy water burns them, or grave dirt prevents them passing, etc.