[M]any factors ... predispose a population to increased or decreased utilization of health care services [that] include: (1) socio-economic status, (2) physician supply, (3) policies and beliefs of a nation, (4) risk behaviours of a population, and (5) health status. (NHSRU, Canada)
As stated above the most significant factors influencing health care utilization are social class status and economic level (whether money and time off from work are available); the supply of available physicians in an area who will or will not accept the available insurances or modes of payment; cultural forces acting upon collective behavior patterns of individuals; and a person's health status, in other words how ill they are and how deliberately they want to seek intervention.
Whether it is the most "important" or not may be debated but the most compelling influence is the socio-economic status of the individual. The consideration of whether money is available and whether time can be sacrificed from work and earnings is the most compelling factor.