From memory, it has much to do with the highest terrain in the country you're flying in. The transition level is set so that any aircraft on the standard 1013.25, flying at the minimum FL, will always miss terrain. The transition altitude is usually 2,000 ft below the TL so that any aircraft operating on local QNH, even if the QNH is at an historic extreme, will be assured of minimum ATC separation from aircraft above it flying at a flight level.
I stand to be corrected.