The main reason is that the Ethereum network is Decentralized, similar to the Bitcoin blockchain network, but Ethereum has several advantages, several years have passed since Ethereum was created and until now, Ethereum has become the choice of many investors because of the smart contracts created by Vitalik.
btw there are many new altcoin projects but all of them are copies of the idea. but due to scalability issues, some developers started to build POA and POS systems on blockchain network, but ethereum is still the choice because it is the king of altcoins.