The support of .xlsx is also important because it is one of the extension which is open to use worldwide. You are right that there are less users of xlsx but if Google wants to give thing in easier way then it is necessary to give give support to all extensions like xlsx, docx, pptx etc.
The *.xlsx format is relatively new. I'm sure it'll be supported in the near future. Lot's of the world hasn't migrated to *.xlsx yet, though, so it was more important to support *.xls. For now, you can save your *.xlsx as *.xls and then upload it.
