I also looked at Misco, Amazon, and Overclockers in the UK and they're all sold out, so my guess would be a combination of higher demand for higher tier and higher margin products combined with unexpected demand for low end Ryzen 3300X, Covid-19, improved yields at TSMC meaning fewer processors are forced to be wasted as lower end products, and the release of the newer processors, such as the 3500X, taking in the chips which were to be wasted as 3100 and 3300X since they binned higher.
Is there a specific reason you want the 3300X over the 3600 other than the fact it's $65 more expensive? This is 2020 and having more than 4 processor cores is useful.