Well, a big chunk of it is just years upon years of exceptional recruiting. The trio of receivers, Henry Ruggs, Jerry Jeudy, and DeVonte Smith, were all at the upper four- to lower five-star range as recruits, and the three played second team with QB Tua Tagovailoa all of 2017 while Jalen Hurts and a bunch of seniors at receiver took all the first team reps. That gave those guys some tremendous chemistry together, and it’s showing so far in 2018.
As for the running backs, Damien Harris came back for his senior season and may well leave as Alabama’s all time leading rusher.