I think there were a lot of factors at play. Like you said everybody kind of abandoned ship when the Blue Jays were coming back down 2 games and even more now down 3-1 in the series. Toronto fans are awful. It goes to show you that the casuals will only show up when the team is winning or playing the Yankees. When this team is down at rock bottom, they'll jump ship right away (evident from seeing less demand for the ALCS).
Secondly, Rogers marked up ticket prices to the postseason way too high for the ALCS. The ALDS was reasonable, though prices in the ALCS are kind of absurd when you think about it. Cheapest 500 level seats face value come out to $111 with Ticketmaster fees. So right off the bat, as a seller on the secondary market its going to be tough to mark them up since it seems like many people don't want to spend close to $300-$400 on a pair of crappy tickets. At least with the ALDS, it was easy to mark up a $60-$70 ticket, since people were willing to spend $200-$250 for a good pair in the 500s. Additionally, the price points for tickets have not been fair and I think someone who bought flex packs should write into the Blue Jays ticket department and complain. Basically a seat in 113D or 130D is the same price point for a seat in 123 for example behind home plate. Why is that? Overall, Rogers took advantage of all the hype and charged astronomical prices for these games. So paying face value was already expensive for the average person, so it would be quite hard to sell tickets at 2x the value, unless you got lucky selling your tickets to some idiot over StubHub weeks in advance.
Additionally, some poster made a good point earlier. People with season tickets and flex packs bought up so many extra seats thinking that they'd be able to sell them at like 3x to 4x the prices to defer the costs for their tickets. So then you see a huge increase of tickets made available in the secondary market, with prices sky high earlier on. Like me and some other posters said, those tickets weren't going to move at those crazy high prices, unless some people got lucky. Tickets weren't moving at those high prices + the Jays coming back to Toronto down 2 games created panic and leading almost everyone dropping their tickets close to face value, which still was even tough to sell as is. Most scalpers I know maybe made $20-$30 over face on each ticket they sold for Game 3 and 4, and even took losses for other tickets. Overall, everyone who got their hands on postseason strips early thought they were going to make a huge profit on their extra postseason tickets, which wasn't the case. This created a flooded market of tickets available with not that high of demand. Also, consider the Rogers Centre holds 50,000 people, it wasn't that tough to get a ticket in comparison to events held at the ACC or smaller ballparks like Wrigley or Fenway.
Lastly, MLB screwed over Toronto with the start time for home games. Only one night game, for 6 home postseason games? That played a huge role. The 12:30 and 4 PM starts made it tough to demand a high premium on tickets since most people work 9-5. This created less demand and more tickets flooding the secondary market. If the Blue Jays played perhaps 1 or 2 more night games (one on a weekend especially), then I'm sure there would have been a higher demand for tickets.