Because the traders in both platform are different. The price increases/decreases because of the traders like us. We are placing buy orders and some other people are placing sell order and vice-versa. That's why a little fluctuations between different exchange is possible. But the spread will not be high. If it happens, people will have the opportunity to have arbitrage on two different exchanges.
On the contrary, market makers are the same players across major exchanges. They're the ones who significantly contribute in keeping the prices between different exchanges more or less similar.
Arbitrageurs are actually one of the reasons why prices will eventually hit a balance between exchanges. Sophisticated arbitrage traders would use advanced systems and bots that immediately detect and take advantage of price discrepancies between exchanges. This results to minimizing the price difference.
