I can't speak for decline in global supplies, but whatever caused it, the MDMA drought of 2009 - 2012ish definitely caused a massive surge in meth use in the local rave scene here in Adelaide, and it seemed to do the same in Sydney (along with GHB) from what I saw when I went over there and, I'm told, Melbourne.
Meth had always been around, but most people used it as a supplemental type drug - take a dab to add some energy to your buzz if you were too munted, or drop half a point (of the old cheap stuff) in a beer to kick the night off, or eat a bit to make it to work after a night out, it was never the main DOC except for a minority who'd mostly been going at it for years, for everyone else it was MDMA, then k/acid, then meth in distant third place. And the meth itself was mostly either powder or gluggy, damp stuff, and almost always just eaten, not the high quality crystal which everyone started smoking.
Then in late 2009 the MDMA vanished and all the pills were suddenly duds/pips/speedbombs. By coincidence, all the K dried up at the same time, and around the same time there was a huge influx of high quality ice (this is when the price doubled, but the purity made up for it).
So if you were going out and wanted something more than booze to catch a buzz and keep you dancing, your choices were basically start using meth, eat awful pills (which were mostly meth anyway, but people would keep trying in the hope that they'd land on a good batch) or trip every weekend (which a dedicated minority did, but weekly tripping is not for everyone). Meth was the closest alternative to pills, and it gave you the energy to keep going, so suddenly most of the rave crowd were smoking it every weekend. It wasn't that people made a conscious choice to swap to meth, it was just the only decent quality drug going around in high quantity.
It really did a number on the scene - the atmosphere degraded massively because people were no longer loved up, attendance dropped as people spent more time in their car passing a pipe around than in the venue, a lot of drama emerged surrounding money and dealing, eventually a lot of them developed a problem and stopped going out entirely, etc etc.
It was pretty tragic to see, honestly.