Maybe still a leftover of pre Excel 2007. From this site Big Table the current limit is 16384 columns. It would be nice if the same limit could be used by Mendix.
I talked with Johan of Mendix about this last week. If you look at the apache poi xlsx is supported and even big tables can be created by streaming. Take a look here and here With some java knowledge you can create your own export, but it would be a lot nicer if Mendix just build this in instead of the old xls export which if I am not mistaken uses the same poi but uses the HSSF instead of the XSSF.