The main reason is FL550 is controlled airspace. Controlled space ends at FL600, the SR-71 tends to stay around FL800 when at mission speed.

Well they still haven’t unclassified the top speed. The only published information is most missions were mach 3.2 or slower. The last flight was allowed to go mach 3.3 and allowed other exceptions. This is from one of Col. Richard Graham’s (retired), Sr-71 pilot and squadron commander. The reason for the Mach 3.3 was…