Money and to differentiate different trim levels. Another issue is that people forget to change them or can't be bothered to get it done so the manufacturer can save a few pennies and then discontinues it. A good example is the Mitsubishi Galant where they discontinued it in the 2003 models but earlier ones had it.
The Versa calls for replacement at 15K intervals and some people do not want to be bothered paying for a cabin filter change regularly.
Frank
2009 Versa S sedan MT-6 with 25200 miles