There is a lot less Alliant powder available compared to Hodgdon powders because their parent, Vista outdoors, is also an ammunition manufacturer.
Hmmmm ......... maybe.
There could be a simpler reason - straightforward supply issues, and as with Hodgdon's Australian imports logistical issues. Alliant's Reloder powders all come from Europe, mostly from Eurenco Bofors in Sweden, and a smaller part from Nitrochemie Wimmins AG in Switzerland. Only Power-Pro ball grades of the rifle products are made domestically (in Florida). Both European manufacturers may well be running flat out and unable to meet increased orders. Eurenco now has a mere two powder factories across all Europe - Bofors in Sweden and PB Clermont in Belgium, the latter only making ball types. (Including all Ramshot powders and at one time maybe some Accurate grades too.) On top of that pair, there are only two others in the canister market - Vihtavuori in Finland and Explosia A S in the Czech Republic (Lovex marque here, some of the range in the US under the Shooters World label).
Two years ago, we in Europe lost most Hodgdon powders and all IMR 'legacy' grades thanks to new health & safety regulations. The demand for those powders switched to domestic production from the four aforementioned plants. Nitrochemie and Viht have been the big winners here, but demand will have increased across all four and it's almost certainly more attractive and profitable to service the domestic markets selling 500g, 1 and 3.5kg cans than to ship it in bulk across the Atlantic. I don't know how Viht is fixed, but I know that as long ago as the last US Obama-era shortages, Nitrochemie told Alliant that it couldn't increase supply much, or add some additional grades that Alliant wanted but which were, and still are, Europe only. As in the US, these manufacturers supply ammunition factories too for both sporting and military products.
Then there are the logistics issues. Blue water shipping transport has been in a capacity crunch for over a year now with few signs yet of easement and shipping rates remain very high globally. When carriers and ships captains are turning business away, explosives and other dangerous good cargoes that often add workload and lots of paper handling, maybe cause docking restrictions and delays aren't going to be their first priority. Lack of explosives goods' capacity on the Sydney, Auckland, US West coast shipping route has been a choke point for Hodgdon's Australian manufactured Extreme grades for many years and I don't suppose recent events have made things any easier.
You in the US are better off than we are in the UK in that you have one surviving domestic canister grade supplier in General Dynamics St. Marks in Florida. Our last factory (ICI Nobel) closed in the late 1980s or early 90s. Otherwise though, you're reliant on long distance imports - Canada (most IMRs and some Accurate grades); Europe (Alliant, Ramshot, Vihtavuori, Norma, Shooters World); Australia (Hodgdon and some IMR extruded grades). I'd imagine the extra demands placed on the US domestic St. Marks facility have been way, way over its ability to increase production.
In a 'normal' supply/demand balanced economy, ongoing unsatisfied demand of this nature would see new entrants and/or existing suppliers investing in additional production capacity. The regulatory and public attitudes constraints on existing explosives plants, never mind opening new ones, mean that far from getting new producers, government and public pressures work the other way to close long established facilities, or place further restrictions on their activities, storage and transport. We live in a risk-averse world these days, in the West anyway.
Finally, to add the icing and cherry on the top, most western governments have pulled out of this business. No state powder factories these days - the private sector will supply and we won't give a penny, cent, or centime towards keeping a 'strategic capacity reserve' is the official attitude. If manufacturers can't supply the current market demands when the West has pulled out even modest conflicts in Afghanistan or suchlike reducing military demands and consumption, imagine what ammunition components supply will be like if we get into even a minor shooting war with Russia or China. I don't know how much coverage Ukraine is getting in the US right now, but it's half the news time on our side of the Atlantic. You know, the little matter of 120,000 Russian Federation troops and a few armoured divisions sitting on the eastern Ukrainian border and people talking about the threat of imminent war in ways not seen since the 'hottest' periods of the cold war.