It comes from ICAO Annex 1
2.3.3.1.2 The applicant shall have completed in aeroplanes
not less than 10 hours of solo flight time appropriate to
the class rating sought, under the supervision of an authorized
flight instructor, including 5 hours of solo cross-country flight
time with at least one cross-country flight totalling not less than
270 km (150 NM) in the course of which full-stop landings at
two different aerodromes shall be made.
Prior to 1999 the UK was happy to accept around 130 nm:
CAP53 -the qualifying cross-country flight during which the applicant made 2 intermediate stops one of which must have been at least 50 nautical miles from the aerodrome of departure. Cross-country routes which fall short of this requirement, but which contain one sector of not less that 50 nautical miles may be accepted. Prior approval to include such a route must be obtained from the Authority (FCL3).
The assumption was probably that a triangle would include three 50 mile legs, few if any did!