Why does Chinese SMS on mobile phone only support 70 characters?

Each short message can send up to 70 bytes. Each short message can send at most 140 bytes (140 characters full of numbers, English letters and English symbols, or 70 characters containing Chinese characters or Chinese punctuation).

This is not the regulation of the operator, but the regulation of the underlying communication protocol. All mobile phones must abide by the same communication protocol to ensure that the sent short messages can be correctly forwarded by the mobile operator's server, and also ensure that the sent short messages can be received by another mobile phone and viewed correctly.