Why not? Because the single greatest threat to Amazon's business continuity is a major quake in Seattle. Vancouver sits on some of the same tectonic plates.
It's likely that this move is, in part, an attempt to address the business continuity risk.
It's likely that this move is, in part, an attempt to address the business continuity risk.
https://en.wikipedia.org/wiki/Canadian_Shield
Not only that but the public transit is undergoing rapid modernization (ie: light rail) and the software industry is spiking.
But there's all that sweet, sweet Rideau Canal beer and poutine.
Aside from that, Ottawa is home to a [small, but] thriving startup and tech community. Prices as compared to SV or Vancouver or Toronto would be lower, while quality of life remains high. Even if it's less exciting. I don't think it's a terrible suggestion. (And I'm saying that from Toronto!)
Ottawa city limits are already huge, so I can't imagine what else qualifies as the "metro area" Kingston perhaps? Or they're counting Gatineau as a separate city but within the metro area?