Because Go made a promise to be backward compatible for Go 1 - Go 1 and the Future of Go Programs - The Go Programming Language
Further clarified in Frequently Asked Questions (FAQ) - The Go Programming Language
Quote:
The APIs may grow, acquiring new packages and features, but not in a way that breaks existing Go 1 code.
Generally, it is an industrial practices to be backward-compatible whenever possible.
Delete - strictly speaking, is a 1-way action where deleted entity are usually unrecoverable, thus breaking the user experience in terms of expectation. Go is used in many critical system worldwide and introduce a small breaking changes without careful planning and communications can be disastrous.