I don't think it's more recent. If anything, it's decreased as scouting becomes more and more systematized. Yet identifying talented players is still an inexact science. Off the top of my head, three of the reasons undrafted free agents can be highly successful in the NFL is:
1. College situation -- coaches misjudged their ability or they were stuck behind another (usually better) player
2. NFL is a different game than college football.
3. Some people are late bloomers physically and/or mentally, i.e., they don't "get it" until later.
From
Gil Brandt's column at nfl.com
Twelve players elected to the Hall of Fame were undrafted free agents out of college, including Warren Moon, one of this year's enshrines. Two of the twelve (Willie Brown and Jim Langer) were cut as rookies in training camp but still ended up being selected.