1. Genetics. If the calf isn't bred to have any hair, then guess what..... he ain't gonna grow it magically.
2. Hard work. Rinse, blow, sheen. Rinse, blow, sheen. Rinse, blow, sheen. Comb, brush, rinse, blow, sheen, comb, brush...... get the idea? Having great hair on a calf is hard. Period, end of story. If you see a calf at a show with phenomenal hair, you can bet your last dollar that it didn't happen by accident. That kid and his parents will NOT tell you "he was under a tree, with good shade." If they do, they're trying to get you to take a shortcut so they can whip your ass again next year.
3. Be smart enough to not screw up a hairy calf. Don't ruin good hair by over-soaping, letting his skin dry out, letting his skin get irritated, etc. Be smart enough to not screw up a good one. There are people out there that don't know what to do, even if they do get a genetically hairy calf. Don't be that guy.
4. If you have the resources & the TIME, then use a cooler. It's dark and cold, and will help grow hair. But, don't think this is the easy way out, because this and step 2 in the equation go hand-in-hand. Coolers are an enormous amount of work, and take an enormous amount of time, and they're just an addition to the rinsing, blowing, and sheening. If you ain't willing to do the rinsing and blowing necessary to have hair, don't bother getting grandpa to burn $15K for you to have the latest greatest coolroom.