I’ve read claims for both. Swept wings certainly do work better for long range, high speed flights. They also allowed the plane to fit in existing hangars.
The original 747's wingspan was 196 feet. The largest civilian aircraft wingspan at the time was the DC-8-63 with a span of 148 feet. Another limiting factor for most hangers is height. The top of the 747's front fuselage section is 32 ft tall. The top of the DC-8-63's front fuselage section is 20 ft tall. For many large aircraft, the tail does not fit into the hanger, to minimize the required size of a hangar.
There were likely very few existing hangars in 1969 the 747 could fit into.
It is unlikely, other than runway and taxiway loading, and runway length, the dimensions of any existing facilities were taken into account in the design of the 747. The 747 was going to change everything, just like the 707 and DC-8 did a decade before (and like the A-380 is doing now). The facilities would have to adapt, and they did. New airport terminals were built to handle the size and passenger load of the 747.
Hangars sizes are rarely a consideration for dimensions of airplanes. When you consider how much a large airliner costs (today a 747 has a list price of $260 million) compared to a hanger (maybe $15 million for a new 747-sized hangar), and the fact there will be many fewer hangars than airplanes, it really does not make much sense to limit the range, capacity, or performance of an airplane just so it can fit into an existing hangar.
There are considerations in the design of new and derivative airplanes for being able to fit into an existing airport terminal gate, which is usually limited by wingspan. But if a aircraft designer is going to try to fit a wing into a given span, he is not going to simply sweep the wings back more. He is going to take into account many things (aircraft size, weight, takeoff performance, speed, range, efficiency, etc.) in the design of the wing.
One case of where a manufacturer designed an aircraft wing to fit into existing facilities was Boeing designing an optional folding wingtip (like the folding wings on Navy carrier aircraft) for the 777. The folding wings would have reduced the span of the 777 to that of a DC-10 when taxiing, parked, or hangared. While offered, no airline purchased this option.