Why did Concorde have a 8000 ft airport elevation limit?
Edit: Concorde can take off and land above 8,000 ft
From October 20-28, 1974, Concorde 02 (F-WTSA) made a tour of the American Pacific coast, visiting London (83 ft), Gander (495 ft), Mexico City (7,316 ft), San Francisco (13 ft), Anchorage (151 ft), Los Angeles (125 ft), Lima (112 ft), Bogotá (8,710 ft), Caracas (236 ft), Las Palmas (78 ft) and Paris (392 ft).

http://www.concordesst.com/history/70s.html

El Dorado International Airport - BOG (Bogotá) has an elevation of 8,710 ft (2,648 m), which puts in 710 feet above the limit of the 8,000 ft maximum altitude. Both runways are 12,467 ft (3,800 m), so capable of handling a take-off.
In addition, Air France flew Concorde twice weekly to Mexico City's Benito Juárez International Airport - MEX (Elevation AMSL 7,316 ft / 2,230 m) via Washington, DC, or New York City, from September 1978 to November 1982.http://www.concordesst.com/history/70s.html
This shows that Concorde was able to schedule regular service to an airport 684 ft lower than the 8,000 ft maximum altitude and visit an airport above 8,000 ft.
I am not sure the 8,000 ft is a hard maximum elevation. It could be for a fully loaded plane. I have not seen any chart to show maximum take-off weight at any altitude.