The closer a coil is to the retainer, the farther it has to travel for each lift event, which increases its effective mass. This increases the force and rate required to maintain control of that coil for a given path or acceleration. This becomes a problem at high rpm, when the spring cannot control itself much less the rest of the valvetrain. By reducing the diameter, you also consequently reduce the effective mass of each coil as you get closer to the top of the spring. This in turn gives better control over the spring coils themselves, and the valvetrain overall.