When you take a look at the source code for the Main Page, you will find the tag ids "mf-" being used. These tags allow Wikis to serve entirely different main pages to mobile users. This is done by marking certain elements for display with id attributes "mf-" or "mp-". When an element is marked in this way it will be shown and all other elements will be hidden.
The straightforward way would be to add this tag to the other elements too. But this is likely to cause weird unintended styling behaviour which will need to be fixed. Probably the reason why it hasn't already been done.
Note that the use of these tags is deprecated (T32405). Alternate ways should be used for mobile versions of the main page. Take the example of how this is done on Wikidata.
Additionally, an alternative method by using class "nomobile" is also available. See Mobile Gateway/Mobile homepage formatting.