Why do only a small percentage of traders consistently make money in the stock market?

Because of lack of discipline.

There might be multiple reasons behind it such as 

• Managing the risk and reward.
• Following strict rules in trading.
• Developing a good trading phycology.
• Having trust on the strategy you use.
• Analyzing the market

Lastly, After all this as mentioned below Knowledge and experience also matters a lot.

So don't give up! Just understand your weakness and work on it. You too will learn and earn one day for sure...

knowledge and experience matter a lot to most people or a small percentage of people are invested in knowledge.

Because there are very little people having the brain to trade.