Re: Using toys as tools...
Firstly I would keep in mind that any essential flight equipment needs to be supplied and maintained by the airline. Using personal-use devices for essential flight procedures ia an automatic fail, whoever the supplier and whatever the device/OS. Using a pilot's personal BYOD as required flight equipment is open to all sorts of problems, whatever the device (though I concede some devices might be less so than others). If it's done properly, any such equipment would be supplied / maintained / configured by the airline and properly locked down and configured.
Keeping that in mind, to address your points:
1) They need a product that will last the trip, even if the charger dies.
If they're using it as a pre-flight data input, not necessary. If needed and essential through all the flight, there should be backup, whether it's an extra charger or extra device. Manufacturer / OS is irrelevant
2) They need a product with a rich and vibrant software ecosystem and numerous developers that are familiar with writing software for that device.
I'm sure there are plenty of developers for both iOS and Windows
3) They need a device that is stable, not rebooting unexpectedly, throwing random driver errors, downloading so many patches it stops working because it filled up primary storage or any of the millions of other reasons why a stock Windows device will bite the dust where an iOS device won't.
AFAIK Windows Surface devices are pretty stable and can be set up to not auto-update, and would be so set up as a corporate device. As I pointed out in the beginning, if it's a personal device that's an automatic fail anyway.
4) They need a device which will work with $external_device or $external_interface; in the mobile world, that always means iOS support.
5) They need a device that anyone can use over the course of generations without retraining.
That's as valid for Windows as it is for iOS
6) They especially need to never have to fear that a future update will completely change the UI, application compatibility or so forth in a radical fashion.
Surely the important thing is that the App and/or program has a consistent interface, not so much the device?
7) They doubly especially need to be able to trust that the device won't apply game-changing updates without permission or snuck in as "important" or "critical" updates.
8) They may have security concerns that require information entered to not be scraped and sent back to the mothership. The exacting details of a plane's takeoff, landing, flight path, etc all seem like things I'd like to keep secret.
Of course, there could be other reasons why iPads are the tablet of choice, my guess is first mover advantage, they started with iPads and kept on with them, the choice was already made long before a suitable Surface device existed (which does not mean that the latest Surface couldn't be used instead).
And to reply to the previous poster asking why not use Surface if easier data entry is required and accidental input is undesirable, I'm pretty sure that keyboards and covers exist for the iPad.
Bottom line, this has NOTHING to do with the device and EVERYTHING to do with incorrect procedures (no double-checking of the inputs) and human error.