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From what I know of the situation (don't trust me, I'm not going to offer citations or sources), this attack wasn't particularly large in terms of gigabits/second. It was, however, very large in terms of economic impact.

I would assume that when a large number of big enterprise-y things go down, HSI takes notice. When other providers get attacks that are 20x larger (gbit/sec), but have much less widespread impact and impact on less enterprise-y things, they don't care so much.