A few options I can think of, though no tools that make this super easy...
A Python script could be a good option if you want to stick with KML all the way through. You could write a script that parses through your file, sorts based on your SimpleData attribute, and applies folders as desired. Probably easiest in Python, due to the available KML libraries and toolsets.
QGIS is another option. It's probably easier, especially if you have some GIS experience and are not into scripting/programming... though you might need to convert to Shapefile format first, do your analysis, and then export back to KML. With QGIS, general steps would be to open the KML (convert to SHP if necessary), confirm that the attribute table imported correctly and looks as you expected, sort or otherwise arrange the table based on your desired attribute, and then export to KML. I don't remember if the KML export tool includes a way to group by folder. If not, then you'll probably want to select each sub-set, export it to a separate KML, then open all the resulting KMLs in Google Earth, put them in a over-arching folder and save that out as one KML file.