No idea, but I suspect the subset of potential customers who need more than four analog inputs and can't be bothered with an external ADC or multiplexer is fleetingly small. If you're serious about analog signals, then you'll need a separate ground anyway, as well as a good reference.
I suppose many of the Arduino crowd's projects, use I/O and not much or any analogue as such. So, I suppose it is not used, that often. It would be interesting to know, the percentage of usage of the PICO, who need or would benefit, from more/better analogue to digital capabilities, especially as regards number of available inputs.
The PICOs seem to be available for sale (in stock) again, now. But, there seems to be a max limit of 1 per order, and you have to add almost £3 to the order for postage. As it becomes more available and in-stock, my options for being able to easily buy one, will increase.
There is a supplier, who allow up to 25 and give free postage if order > £25, but they don't have them currently in immediate stock (seems to say >= from 8th February ), but they seem to have partly poor user feedback as sellers, so I'd prefer not to deal with them.
I expect to get at least one, but not at the moment. Ideally, I'm hoping the max of 1 per customer/order limits will be removed and/or more convenient sellers, such as Amazon and ebay, have them economically priced and available for sale. (As with other things, at sky high prices, such as, very approximately £15/£20 each, you can get them from lone individual sellers, on ebay/amazon. Which is a ripoff price.
If the maximum order limit of 1, is NOT removed, later. E.g. It doesn't seem to have been lifted with the Raspberry PI Zero things. That could be problematic, if anyone wants to use it, in a project that gets sold, small scale, and/or needs many boards to work.
E.g. A big LED display controller setup, with 10 or 20 boards, spread around, to control the flash patterns on the various banks of Leds.
I suspect/hope the buying limits, are only temporary.