I'm a manager of a network where the standard browser is IE11. Just three simple reasons:
1) In the early days of Windows 10, when Edge came out and was new and exciting, it was immediately obvious that little had been done to make the kind of in-depth control available in IE, accessible, or exposable, even to advanced users. Features had been stripped out. "Power users" whose preferred/required workflow involves large tab counts (we have those, and it's a deep personal preference apparently) found Edge less capable/efficient. Users who expressly *didn't* want to save web pages as PDF didn't have a choice. Some of that got added back in by extensions, but the thing about extensions is, they aren't guaranteed to be supported. Nobody actually liked it, compared to IE, and that memory of early stage complete let-down is probably persistent.
2) Edge was bundled with Windows 10 only. Windows 10 changes daily, because it's "a service". It breaches peoples privacy to the point that we had to put in custom firewall rules and system policies to block telemetry entirely, because under W10 it became so much *more* pervasive (speech to text being sent to yourselves, who can tell what else) that it's not actually possible to tell what personal data is being sent and prevent it neatly within the system. That's a whole other issue, but is strongly if indirectly relevant to Edge migration whatever ones view. To underline the point, before W10 there wasn't a mass "block telemetry" issue, with W10 there was. Service packs were unreliable. Drivers couldn't be blocked even if things broke. (Remember the old sysadmin adage about not upgrading until SP1 came out? Case in point)
3) Microsoft built in their own browser (of course, but still), which meant that users who stuck with W <= 8.1 also stuck with IE. And as it works for them, they'll probably keep sticking with it.
That combination of constant unpredictable change/instability, constant breakage, chasing "new" in preference to "solid" or "stable", indifference to privacy (telemetry still can't be cut off completely from within any usual version of Windows or Server) and an air of indifference to the issues it caused, led a significant number of companies and individuals to decide to avoid W10 as far as possible. Which meant, essentially, avoiding Edge. The idea that Edge was exclusive to W10, probably intended as an upgrade incentive, backfired for these people, as instead of deciding "Want Edge -> Get W10", they decided "Don't want/need Edge either, or enough to get W10 -> IE is good enough". The existing OS has an existing browser... "why use unfamiliar" goes the logic.
As a final thought, IE is only problematic because of Microsoft's own past decisions to build it so. IE incompatibility and Microsoft's lengthy policy of "embrace, extend, extinguish" applied to the browser and other matters, were notoriously features of Microsoft WIndows policy for many years. Edge is living, in a way, with their long term consequences.
Edge essentially failed because of Microsoft's own choices - to make an OS with features that deterred a significant group of people, a browser that was said to be new and shiny but had major disillusioning gaps for those used to browsers, and to try and associate them exclusively, so that the exclusive Edge would act as leverage to migrate users to the new OS, whereas for a substantial group of users, the issues with OS and Edge alike acted as a deterrent to avoid both.
Microsoft mismanaged Edge in the early days, and that's still the burden it's living with. IE is mainly a problem because of Microsoft's own direct choices in the past and its decisions about how IE should behave. The matters that were problems then for those users will still be problems now.
I'm not actually sure *how* you work around that kind of millstone.