There are, at least, a few reasons for this.
- Apartments generate passive income.
- You only need to worry about the maintenance of one building while still collecting many rent payments.
- It's getting more difficult to find great investment deals with single family homes--since so many investors are looking for them now. Fewer people are looking for apartments; so theoretically more deals can be found with less searching effort.
Here is the link to the article "No McMansions for Millennials"
http://realestate.yahoo.com/promo/no-mcmansions-for-millennials.html
It does not address investing; however, I think there are investing-oriented takeaways from this article.