There were many reasons Notes died.

It was very hard to get data in and out it had almost no capability for data import/export.

Internet email killed Notes early advantage as one of the first email systems.

It was a very closed environment hard to connect or program outside its own sandbox.

Sharepoint was a full on assault by Microsoft on the groupware category and its enormous success was at the expense of Notes.

The web did many things better than notes there much much overlap.

The UI was clunky in some ways.

Some of the concepts like replication were just too much too early for many people to grasp.

SQL rose in the corporate world chipping away further at notes.

The Notes formula language was good ish for the time but really became very dated, and the alternative LotusScript was a dead end too.

Unstructured document databases were very polarizing sine people hated them with a passion.

The parent company Lotus main product 1-2-3 which ad dominated the spreadsheet world got smashed by Excel.

There’s more reasons too but there’s enough there you can see the doom of Notes.