I can't speak to crack but cocaine in the US went through a boom primarily because the supply and logistics became more sophisticated which meant that more was available. A lot of this, in my view, has to do with the evolution of the marijuana trade in the 60s and 70s.

Marijuana of course becomes incredibly popular with the rise of counterculture, and anyone with seeds can grow it, however as any old hippie or college kid will tell you growing good pot is a completely different story. The best product of the time comes from South America, and by the 70s demand for it is such that farmers and US-based smugglers are using sophisticated networks including trucks, aircraft, ships, etc to move their stuff.

By the end of the 1970s the counterculture movement has faded and, more importantly, information about how to home-grow has become (relatively) more widespread, as have the seeds from Colombian and Panamanian strains. The giant smuggling networks of Robert Platshorn's "Black Tuna Gang" and the like are no longer necessary or profitable as far as pot is concerned.

Enter cocaine. Previously a niche drug of sorts, known by most but far from a drug of choice to the hippie set. Coke offers some advantages to the cartels that pot doesn't: namely, easier to control and monopolize supply. Coca is not amenable to growth outside of its natural habitat of South America's mountainous regions, and the processing from leaf to powder is an involved task that takes more specialized knowledge than growing pot does. Because it's a powder, it's easier to compress into bricks meaning more can be loaded for transport; coke also can be and is cut with various fillers, meaning the supplier can stretch his stock more and turn more profit.

These and many other factors make cocaine a lucrative prospect for the drug cartels and growers, many of whom participated in the pot trade in some capacity previously. Using the well-trodden paths from Medellin to Miami, their product moves in in bulk. Coinciding with this is a shift in fashion and pop culture in the 80s; specifically, the evolution of music and nightclubs. Where a nightclub of the 60s and 70s might've played blues or jazz, the sort of music you sit back and enjoy with a relaxing substance like alcohol or pot, the early 80s brings with it disco and electronica, active music that emphasizes dancing and energy. The club drug scene shifts accordingly to stimulants, cocaine chief among them, and from there the scene takes off.