Last September, I asked the question "why do you think SF RPGs are less popular than fantasy RPGs?" Today, I'm going to go a step further and ask, "why do you think modern day RPGs are even less popular than science fiction ones?" For the purposes of this question, "modern day RPGs" includes any game whose setting is post-medieval and includes no clear magic or supernatural elements. Consequently, a game like Call of Cthulhu is excluded from the category, because of its supernatural element, even though it's set in the right time period.
For myself, I think modern day RPGs are a lot less appealing than either fantasy or science fiction because, when devoid of magic or other similar things, they feel too much like the real world and most gamers roleplay to get away from the real world, even for a little while. When I look back on my own gaming history, it's true that I have played a handful of no-magic modern day games with some devotion -- Gangbusters comes immediately to mind -- I never played them as much as I've played, say, Dungeons & Dragons or Traveller. And, much as highly as I continue to regard some of those modern day RPGs, I can't deny that my interest in picking them up again now is more limited than in playing more fantastical games.