The open source selling point is also irrelevant to most people outside of HN.
1. Network effects. All your friends are already on slack/discord.
2. Difficulty to get started. Simply understanding what Matrix does, especially for someone non-technical, is more complex than setting up an entire slack workspace and inviting your friends.
3. Clients. While the slack client is a resource hog, for most people it is better than Riot/Element or other 3rd party options such as weechat.
I think all of these things are improving and Matrix absolutely will gain ground, but it will take time.
For example, a Slack instance can be configured to log every conversation (including DMs) for audit purposes by company admins. Matrix enforces e2e encryption for peer-to-peer conversations and I believe disabling room encryption requires code changes.
For those who are legally required to be on record, there are other ways to keep track of the conversations for audit purposes without compromising the e2e encryption. For example, every room could have an audit bot invited by default, visible by the users, and which would record everything being said. Then you can setup the access to the logs from the audit bot to only be unencrypted in certain conditions, e.g. if the 2 halves of a key giving access to the account are put together. It's secure, clear for the users and legally compliant.
[disclaimer: I'm from the Element team]
It all sounds very complicated compared to paying a bit of money and toggling a setting. For example, googling for "elements/riot/matrix audit bot" results in no pertinent results from what I can tell. Being possible is not the same thing as being easy to use.
edit: Also companies don't care about being clear to users except as legally required or beneficial to the company. Employees not being constantly aware that they're being watched all the time is a positive and not a negative.