It's the community and the tech.

The tech enables developers to build powerful applications.

The large community of developers building apps attracts the large community of users using them. The large community of users attracts developers to build apps, which attracts users...and so on and so on.

Communities and networks are difficult to reproduce and migrate. This is why the most successful alternative chains have bridges to Ethereum and are compatible with the EVM so that all the same apps and tools also work on their network.

Ethereum is at the center of all of this.