Can anyone explain or point to any resources that explain why the Leopard is the tank of choice for Ukraine over, say, the Challenger or the Abrahams or even Russian tanks former Warsaw pact countries?
Ex-Warsaw Pact stocks are increasingly exhausted and what's left is either still needed by the potential donor or is likely so rotten as to be a liability rather than useful. Though there are probably still some knocking around. Eitherway they don't bring a qualitiive advantage over the Russian stuff and, in some respects when the Russian kit is as advertised, are worse. A T-90 is tougher than a T-72 or T-64 which is a lot of What Ukraine already operates or has received as donations.
Challenger 2 is an excellent MBT (though long in the tooth in some areas) but there are precious few of them. There were only ever slightly under 400 made for the British Army of which a little over 200 remain operational. It's safe to assume that the rest have been stripped for spares and/or have rotted away and would need significant work to restore to operational condition. Of the roughly 200 that still might be operational a decent chunk of those are probably "operational" and themselves would need work. The British Army was only expecting to be allowed to convert 148 from Challenger 2 to Challenger 3 so I suspect the vehicles over that number are probably increasingly having been put out to pasture. Fundamentally it remains an excellent MBT but there are so few of them that unless we up and give Ukraine our entire tank fleet we'll never be able to give enough to make a real difference.
Abrams is also an excellent MBT (the latest versions will be better than Challenger 2) and it is available in large numbers. The Lima Tank Plant in Ohio is still churning the things out even though the US Army has more than it knows what to do with thanks to Congress continuing to buy tanks each year. There are thousands of the damn things parked up in the deserts out in the western US that probably just need refurbishment and could roll into battle. But there are issues. Logistics are somewhat tricky. Whilst some people have gotten fixated on fuel (and fuel usage) this is a red herring. The US Army powers their Abrams using jet fuel because it's logistically simpler for them to do so. But the engine is a multifuel engine. If its liquid and burns an Abrams can probably run on it. It can certainly run on the Ukrainian fuel of choice which is diesel. Equally whilst its a complex beasty I think it's unwise to doubt the capabilities of Ukrainian maintenance personnel at this point. No the main issue would be that if you need spare parts, especially spares which you cannot easily manufacture locally, then you're having to rely on the US logistical tail running through Germany and back to the continental United States.
Another issue, which I've not seen much discussion of, is that there are legal export controls which prevent (in theory) the US government sending US Army specification Abrams to overseas users. Previous expert versions have had to have an entirely different turret design which lacks a lot of the Gucci electronics and armour features (it's one of the reasons why it's easy to find videos and pictures of destroyed Iraqi Abrams). This means that in order to send them Ukraine, assuming no exemption is forthcoming and just look at the state of the Republican controlled House of Representatives, new turrets are required. This means it will take time. Lots of it.
Which leaves the Leopard 2. Leo 2 is also an excellent MBT (keyboard warriors love debating which is the best MBT, reality is you'd be happy with any of these three to be quite honest and the deciding factors are likely to be things out with the tank itself!). Leo 2 has the advantage of there being lots of them lying around (hundreds within Europe of varying types) so whilst each donor has to weigh the loss of their own tank force whilst seeking replacements if ever operator donates a small number that will soon add up to quite a decent amount for Ukraine. Logistically it's simpler. Poland and Germany all operate them and are geographically close to Ukraine meaning it's quicker and easier to get spares to and from Ukraine. Training can be done in Poland (and is allegedly already starting) by Ukrainian crews. Now that Germany has finally be kicked into doing the right thing there aren't export controls to worry about so they can be provided pretty much as is. It just ticks a lot of boxes whilst Abrams and Challenger 2 both tick lots of the same boxes but just not necessarily as many as Leo 2 does.