The safety position has changed over the years as new schemes and trends have evolved. Most programs used to recruit big, physical safeties that would intimidate receivers over the middle and were especially effective in the run game. These guys were almost like having another linebacker on the field.
However, with the ever increasing use of multi-receiver sets in today's college game, this type of defensive safety became a liability in passing situations. As a result, the qualities of a safety have come full circle and now programs are looking for more athletic, "range" type players. They still must be able to hit and come up vs. the run, but their ability to break down and tackle in the open field is very important. Many guys can create a collision that looks good, but if they can't break down and make a key tackle in the open field, they will hurt a defense.