There's several factors. One is that the 1978 decriminalisation at federal level didn't mean everyone could start homebrewing, let alone going commercial - there were still all sorts of inhibitors at state level, some of which were not sorted out until the late 2010s. For instance, in some states you couldn't sell beer at the brewery without first selling it to a distributor (who took a handsome cut) and then buying it back off the distributor, even if it never left the premises. And just generally licensing breweries could be really tough as local planning people had a lot of latitude to make rules up on the spot. It's hard for non-USians to appreciate just how decentralised the US is when it comes to this kind of stuff, so decriminalisation at federal level is not the same as something being decriminalised by Westminster or Paris.
The other thing is that it takes time to develop a culture, particularly since there was no internet back then - they were dependent on a few books coming out of Britain and there was no infrastructure for the retail of homebrew supplies, which is perhaps one reason things really started in the Pacific Northwest where they had direct contact with the farmers - Bert Grant in Yakima being perhaps the most famous example.
You did get one or two in the early days - Jack McAuliffe's New Albion is generally regarded as the first microbrewery of the modern era opening in 1976, and Sierra Nevada brewed its first gyle in 1980. But it's notable how many of the early 1980s brewers picked up the bug through some connection with the UK - McAuliffe credits being stationed at the submarine base at Holy Loch as being the starting point for his journey into brewing. And I guess that even as that initial trickle of microbreweries started up, they were being matched by existing breweries closing or being taken over.
