There’s a line drawing problem. If strikethrough is added to the core, why not cite, mark, spoilers, tables, etc as well? The line has to be drawn somewhere, and it appears to have been drawn at “features found in John Gruber’s Markdown” (where fenced code blocks are counted as the same type of feature as whitespace codeblocks). A lot of work has already gone into working out a solid spec for just these features. It would take even more time if everyone’s favourite feature was also included.
That is a decision of the app developer, not the spec writer. Even for features included the core, the developer may still choose to exclude them. For example, as a developer I sometimes disallow images and headings because I want to ensure that the writer only uses a minimal set of tags. If you’re frustrated, you should ask the app developer to enable your desired feature.