Because target shooters happen to want good BCs in addition to accurate, consistent bullets, so target bullets have both features (consistency & good BCs).
I don't think that either is more intrinsically accurate - they are the same in a vacuum - accuracy is a function of consistency / uniformity, and lack of eccentricity / asymmetry; not really a function of base design.
All of above: IINM.... I could be wrong. It's possible that boattails are more accurate instrinsically - so take my "pronouncements" with a grain of salt.