I suppose you’re not using the Excel exporter module, but rather the built-in feature of a Data Grid widget. Unfortunately, when the grid data source is a microflow, the grid will only export the current page to Excel. It does not even make a network request when you export.
To keep using the built-in Excel export you need to create a persistent entity and populate this entity with your results in the microflow. Then make this entity the Data Grid data source.
Alternatively, you can install the Excel exporter module and add an regular action button to call the export microflow. However, you will have to manually configure an excel template.