If we look back many years, there was not really any standardized boarding procedure.
Many DC-3's had the pax entry door on the RH
side, as did many CV240's.
As aircraft got bigger, and cargo loading took longer...and longer (belly cargo is profitable), the standardized way pax were boarded was thru L1, to keep them away from aircraft service vehicles. In addition, single point refueling is many times carried out on the RH
On TriStar aircraft, oftentimes boarding was also carried out thru L3 as well.
A300 aircraft will oftentimes deplane thru both L1 and L4
at the same time.
In addition, at many airports (especially older airports), jetway availability and location dictate boarding procedures.