@Def - case insensitive file systems
Most of the users I have met have difficulty typing file name at all. They click on file names, so there is no issue with them typing them with the wrong case. In the unlikely event that a user ever types a file name, the mail user agent or word processor or whatever could do a case insensitive search if a case sensitive search fails. By all means, put such functionality into the file selector of whatever tool GUI tool kit you like so applications behave consistently. All of this can work fine, without the file system driver knowing a thing about unicode.
Now take a look at what happens when some utterly clueless PHB says that the file system driver has to do case insensitive matching. For example 'ǳ'. If your browser and font system are reasonably modern, that example should look like 'dz', but if you try to select just the d or z, you should get none or both at once because ǳ is a single letter. If you capitalise a whole word that includes ǳ, you need a Ǳ. If you only want initial capitals then you need ǲ. Things go rapidly down hill when you come across ǆ, ʥ, ʤ and ʣ (look closely and you will see the letters are closer together in ʣ than in ǳ). Unicode has plenty of stuff like this, and the number of corner cases grows with each version.
Outside the Microsoft ghetto, operating systems can handle dozens of different file systems. Putting this crap into every file system driver would be insane. Even worse, when a file system driver updates to a new version of unicode, some things that used to match will stop matching and other previously distinct names will match. Piles of automated software that used to work fine will start breaking depending on the file system in use, the version of its driver and the language used to name files.
Years ago, Microsoft software put the clocks back an hour at the end of daylight saving time. Because Microsoft thought is was a good idea for the system time to be the same as local time, an hour later they put the clocks back again, and again... That bit of stupidity caused a day of pandemonium in each country that uses daylight saving time until the problem was fixed. On the plus side, the failures were sufficiently widespread and synchronous to hit the news so people understood what was going on, and how to deal with it. Case insensitive file system drivers problems do not hit entire countries on the same day, so they do not make the news. There are still people out there who do not understand why that badly designed feature is such a can of worms.