Moving from XLS to XLSX will break your macros and would require all machines to be running Office 2007 or later.

Compatibility. It's reasonable to predict that some of the participating entities are running software that is at least a decade old.

If somebody finds it easier to do this process using Excel than with a scripting language, I wouldn't expect them to know the differences between file formats. I would be surprised if they understood the concepts behind file formats.

They may have chosen XLS because they used Excel back when that was default, and now they don't want to "risk" anything by switching horses mid-stream.

Inertia, most likely. They were using XLS 10 years ago and never saw a reason to change.