Amazon doesn't really have a lot of reason to pick it up. They have the full show regardless, and they wouldn't get a whole lot more new customers than already watch the show there and on TV.
The whole reason it got cancelled is the network eventually concluded the viewers they regularly got weren't enough to justify the price paid for the show, and unless it's bought for basically 6 figures it's not going to get enough viewers that wouldn't be a disappointment. It was an incredible niche program, but still a niche program.
It's like the Twitter trend. Millions were tweeting about it for hours, but at the end of the day, where were those millions when they were needed and it mattered most (watching the program on time)?
They were given a rare gift, but they didn't want it until it was gone.
The incentive for Amazon, I would suppose, is that while the show doesn't have a massive audience in the ratings, it has a very active and devoted fanbase in addition to a lot of critical appeal. Amazon's Transparent received quite a lot of coverage, and has been very highly praised, but their offering otherwise is pretty weak, and not very well known (I actually don't know of any other shows they have outside of Transparent). The goal, if they manage to keep a diverse selection of relatively niche appeal shows with high critical reception, is that they will be seen as a provider who appreciate quality shows and allow for a deal of leniency to creators so long as they are highly regarded. Simultaneously, there is the hope that viewers will recognise this, and subscribe as only on Amazon can they view such a diverse, unique, and high quality range of shows as they couldn't survive elsewhere. In doing this, content providers turn towards Amazon, and then they can attract much bigger shows with a greater appeal to viewers, and it creates an environment that they would hope would be self-sustaining.
Whether or not this is realistic or economically feasible is another thing, but had they secured Hannibal they would be securing it for the critical appeal, and for the message it would send to other content providers (that they value quality programming) more than it would be for the viewers Hannibal itself would attract (that obviously is a factor, but I don't think it would necessarily be the major factor in this hypothetical given that it has so few viewers).