Because the Ping Of Death was an RCE. If you sent _just the right_ kind of malformed or too large packet - and you cleaned up after yourself - you suddenly had a system where you could basically ask any computer you could see to do whatever you wanted, and it would do that for you and then quietly go on its way.
I was temping for Global Affairs Canada in the late 90s, then called DFAIT; I got to hang with some old-school-then, semi-retired CSIS sigint guys.
They thought the internet was great.