Metra Electric uses High Level Platforms.
Its because of the way equipment rosters were in the early days, and its easier to follow the same path, rather then rebuilding everything and starting over
Remember, Metra's roots are from former railroad commuter operations. They used basically standard car types, minus a few small changes (HEP comes to mind, and the later addition of Push-Pull Service) so these cars were set up for low level boarding. When Metra Took over operations, all the stations, not to mention the equipment they got, were all low level boarding, so it has been easier to follow this example, rather then starting over.
The exception to this rule is the Electric District. when the line was electrified by IC, they bought special cars (they had to) and incorporated high level boarding, which on an electric car is honestly the better way to do things, because it leave the entire underbody clear for various electrical components, without having to make room for step wells.