dunno, but probaby down to that old engineering answer…compromise….high bypass ratio are cool as they give decent SFC (specific fuel consumption for those that dont know) and a good whack of thrust from the cool bypass air. The rest goes into the combustion/turbine stage to give ‘hot’ thrust and run the compressor of course.
My old tutor said it was easier to compress a cold gas than a hot gas! Bless him….anyhoo, the design will have a load of factors, not least of which are installed thrust out put, customer bleed requirements and the design of the blades/stages etc come in to……having said all this, not sure this is a historic subject?!