* customers might have complained that the site didn't work on mobile when the desktop site was requested; this may have led to a burden on their support team, and a manager might have decided to block usage instead of fielding support requests
* a team internally might have been spending a lot of time chasing down bugs for desktop on mobile, and a manager decided to officially not support desktop on mobile; instead of just stopping support or carefully triaging tickets (e.g. distinguishing desktop on mobile from desktop), they just decided to block desktop on mobile access
* a team might have been burdened by being required to fix all bugs that came in; instead of dealing with the problem properly (e.g. continuing to allow desktop on mobile but not officially supporting it), they officially killed support to avoid having to work on it
* there could be some KPM to reduce bug reports over some period of time; they killed access to the non-supported use-case of desktop on mobile to make numbers look better
Not really a smart decision in any of these cases, kind of stupid, honestly; but not really nefarious either and the kind of decision that can be made sometimes at a larger place with politics.