Why is there no multiple or block comment in Ada?

The Ada 83 Rationale Section 2.1 says

No form of embedded comments (within a line of text) is provided, as their utility is insufficient to justify the extra complexity. Single comments that are larger than one line are not provided. Such comments would require a closing comment delimiter and this would again raise the dangers associated with the (unintentional) omission of the closing delimiter: entire sections of a program could be ignored by the compiler without the programmer realizing it, so that the program would not mean what he thinks. Long comments can be written as a succession of single line comments, thus combining elegance with safety.

Of course modern IDEs will colourise comments, but you might not notice, and the compiler certainly won’t.