And integration...
The main thing that won Microsoft a lot of migrations was integration, especially with AD but also to a lesser extent with Office. They used that to claim a lower TCO.
The Office integrations were merely niceties in Outlook, like re-using Word for email editing so it was a little more familiar, and having a few Send To type options in Office applications.
Integration with Active Directory was always touted as a significant lowering of TCO. And to some extent it was - by comparison Notes had ID files to distribute, and although IBM did fix that with the ID Vault it didn't arrive until version 7(?). By that time most companies had made their own arrangements, so it was a bit too little too late.
I've met plenty of people who have "managed an email system" on their CV because they created and deleted mailboxes on Exchange. They know nothing about mail routing, or the SMTP protocol, or email headers. They wouldn't know what SPF was if it tap danced on top of a piano whilst singing "SPF is here again" and waving a sign that says "I am SPF", but that AD integration is all they need to claim being an email administrator.
I can certainly make an argument that the day to day TCO of Exchange Server is lower than Lotus Notes. However, I also know what the cost of doing an upgrade for each is, and whilst Notes is easy to upgrade - it's a trivial install over the top of the existing server software, then a rollout of new mail templates at your leisure - Exchange Server is horrifically expensive to upgrade
You have to specify and purchase new hardware, and deploy it. Microsoft has almost always changed something in the storage layer that means research and training, and you effectively end up building a new infrastructure and then migrating your mailboxes and applications to use that infrastructure. This requires more knowledge than just creating/deleting mailboxes, so often organisations are ill equipped for the project.
You can upgrade to the latest version of Notes in an afternoon. Exchange will take you many, many months. So overall I'd say that the TCO is probably about the same, but it's distributed very differently for each product. Notes has a consistent but slightly higher TCO with an occasional mild increase for upgrades, whereas Exchange has a slightly lower TCO with mountainous increases at the start/end of each product lifecycle.
(This is why most on-prem Exchange instances are ancient, and why so many people are happy to move to O365 - why not let Microsoft do the mountaineering for you?)
Integration with AD, and its lower day to day TCO, were a definite factor. Combined with the FUD, it's what got Exchange into its dominant position.