It's tradition, but it also might be a factor for resume scanning programs to see whether candidates are local. (thejobnetwork.com)
Some employers use scanning programs to locate the best resumes and reject those that don't meet minimal standards. (work.chron.com)
The exception here being that you can only enroll a single pair of eyes in the iris scanning program, while you can register up to 10 fingerprints in the fingerprint scanning system. (androidheadlines.com)