Why did they retire the SR 71 Blackbird?

They had a few reasons. Prime among them is the government decided satellites could provide all the overhead coverage you'd ever need. Satellites have a big problem: you can't put them where you need them when you need them. After they brought the SR-71 back to service, it then became a question of money. NOTHING on an SR-71 fits anything else in the fleet. If you need a new part you have to make it by hand. The fuel only works in the SR-71. The tires are special. There's no onboard datalink so there's no real-time collection capability, and that plane is so fast you could never put a datalink system on one. Now that we have unmanned aerial systems, we don't need the SR-71.