Because, in addition to authorization for the aircraft to deviate left or right of course, a VFR climb would have been the proper move. Not a vector below the MIA, and especially not when the closest obstacle is less than 5 miles from the aircraft. Read Ben's account again, he is from the area and knows it well. Your plan of turning an aircraft around after entering the MIA is not just wrong, but dangerous.
Do you understand the rules for climbing through a higher MIA?
A controller is authorized to allow an aircraft to enter a higher MIA provided that: the aircraft's mode C reads an altitude at or above the highest obstacle (it did), the aircraft is on a route of flight that will provide lateral separation from the obstacle (it was), and the aircraft is climbing to the new MIA (it was assigned that altitude, so yes).
Until the pilot reported unable to climb, there wasn't an issue. It was only after the pilot said he couldn't climb that there was an issue.
First of all, read the accident report. The aircraft lost radar contact, and the controller knew about the outage and that contact would resume shortly. The controller issued 16,000 to the aircraft. We can assume that because once he reacquired the target, he asked the pilot to verify he was still climbing. Of course, you disagree but until we have transcripts of audio that's where we stand. You see, contact was lost before the aircraft was near the new MIA. The last moe C readout showed an altitude above the next peak (13,800), so the controller was good on the climb provided the aircraft kept climbing. It was reacquired after the aircraft was inside the higher MIA. Issuing a vector would be the worst move to make. You want to be the controller that turned an airplane into a mountain?
The proper move would have been to offer a VFR pilot discretion climb to 16,000. In addition, authorize left and right deviations from course to allow the aircraft to maneuver around the peaks, hopefully avoiding the mountain wave.
"Climb VFR at pilot discretion, maintain 16,000. Deviations left and right of course approved, when able proceed direct RIW."
Don't you have some kids to go yell at for walking on your lawn?