It's the formations and tactics used to expose defenses. Today's popular footy is wide play, spreading the defense out to open space and bring a numerical advantage.
For a while, it was the bombing outside backs that caused all the problems. Now you have a 4-3-3 where the defense is forced to pit man to man, then the center mids can loop around, or the outside back can overlap to make things rough.
Additionally, the longball game, where the center forward was king, is almost gone in top football today. Smart teams balance possession and counter-attacking with any broken lines being hit with a runner through over just banging it forward and running for it, or holding it up.
Center forwards have their place still today, they are just accompanied more by wingers.
There have been many top centre-forward in the past, such as Batistuta, Ronaldo, Vieri, Drogba, Thierry Henry, Shevchenko, Shearer, Bierhoff. They can do anything in the box. Why are top centers so rare these days? The current forwards are all wingers, shadow strikers and so on, they mostly survive by running around and tearing up the defensive line, is that because the defense is tighter, or is there no such talented centre forward?