Because it's easier to program, I suppose.  If they program it to allow multiple boxes to be checked, they'd need to build the logic to reduce the font size and create statements to list the titles that don't fit.  You could raise this with Intuit but doubt it'd rank very high for future development.

You may want to select the title that best fits their core duties, the more critical role, etc.  If you must, attach a statement to supplement the details reported in Part VII.