Yeah, it must be initialized in the constructor.

Similar to the JVM, the CLR can offer binary compatibility of libraries, which is something Microsoft cares a great deal about. Changing a public readonly field in to a public readonly property is source-compatible, but not binary-compatible. That's why properties are generally preferred, even when you don't strictly need one. The language was actively discouraging either immutability or binary-compatibility by making it more verbose to get a trivial readonly property.