Here's why:
* Modern describes something that is up-to-date, current, or of recent origin.
* It's not a word that describes degrees of something. For example, you can't say something is "more modern" than something else.
If you want to convey a similar meaning but with a sense of comparison, you could use words like:
* More recent (comparative)
* Most recent (superlative)
* Newer (comparative)
* Newest (superlative)
* More contemporary (comparative)
* Most contemporary (superlative)