Each of them is trying to replace BTC in some way, so they are constantly improving in all different issues that BTC really has (such as transaction speed, throughput, and drawbacks of PoW algorithm as well). If you are really interested in technical details, you may want to read about different generations of blockchain, as it seems to me like a right topic to start from.
Definitely you're right thats the main reason why altcoin has its greatest potential also in the market.. Because everyone or every projects aim to surpass Bitcoin in the future thats why until now they are finding a way in order to develop more.. That goals makes altcoin great.. But still until today Bitcoin is on the top...