I reckon it's a combo of a few things. Mostly for this one, it had both the business and consumer sides of the equation kind of line up at the same time.

On the business side, one of the biggest factors was we'd just come out the other side of a recession. This meant interest rates were low and there were a load of people looking for new jobs/businesses and there were a lot of cheap places to set up that business. This naturally meant there were all sorts of new businesses being created around that time.

One such boom was small batch alcohol, because ultimately if you're good at it, you stand to turn relatively cheap ingredients into quite a bit of money. You'll probably notice modern craft beer really kicked up a gear around the same time.

Interestingly for this one, a lot of the early craft gin places didn't really set out to be gin-makers, they want(ed) to make whisky. The problem with whisky is you can't even call it whisky unless it's at least 3(?) years old, and it's not really going to be any good unless it's at least 2-3x that. So what do these distilleries do whilst they're waiting for their whisky to age and hoping they don't go bust? They make quicker to produce spirits such as gin!

Now, small batch is more expensive to make than the industrialised big brands, but gin has the benefit that there's a lot more freedom for the maker to mess around with the flavour. This allowed gin-makers to make their gins stand out against the competition and the sector was born.

There were also a few law changes around the world in a few places that made this kind of business a bit easier to set up.

Now on the consumer side there was this big resurgence in cocktail culture going on. So you've got all these interesting new flavours coming out of these small batch distilleries, what are bartenders who want to twist an old recipe into something new going to go for? It's going to be these cool new gins. Less cutting edge bars want to seem like the cutting edge ones so they start stocking the same spirits. As you'd imagine this creates a feedback loop that means the gins become successful and you see them behind more bars and on more shelves. After a while it's just kinda everywhere

All that, plus gin drinks just make some tasty cocktails and easy mixed drinks

(Sorry that was longer than I expected when I started)