Why is Intel SGX a requirement?

The SKALE Network makes use of certain features in the Intel SGX chipset in order to offer enhanced security and added data protections (the virtualized nature of nodes is also able to leverage the multiple independent Intel SGX-enabled CPUs that the chipsets offer).
A further reason for the Intel SGX requirement is the heavy use of the BLS (Boneh–Lynn–Shacham) cryptography as part of its technical offerings. For example, interchain messaging is powered by BLS threshold signatures. Each sidechain also supports BLS Rollups which provides an efficient and secure way to use the SKALE Network to improve throughput and lower gas costs on the Ethereum mainnet (rollup can generally be defined as a solution where transactions are published on chain, but computation and storage of transaction results is done differently to save gas).