Why does the F-35 Lightning II have only 1 crew when the F/A-18 Hornet had 2?
Why does the F-35 Lightning II have only 1 crew when the F/A-18 Hornet had 2?


According to program officials involved in the development of the F-35, the F-35 is a single seat aircraft because:


  New advances in avionics and sensor fusion technology, which can 
  allow  a single pilot to perform missions that previously required two
  crew members.



Given the fact that the F-35 is build around the concept of
Network Centric Warfare there is less cockpit workload for most strike missions. Consequently, the need for a co-pilot is a lot less. 
The single-seat configuration also maximizes commonality between the 
three variants of the F-35 thereby keeping development costs low.
The survivability of the F-35 includes limiting the probability of   detection. Ergo, a two-seater version can very easily increase the RCS of the F-35 thereby compromising its stealth capabilities


West 2014: Joint Strike Fighter: What Do the Pilots Who Are Flying It Today Have to Say?

That said, one major drawback that remains on the one seat design is the demand on one pilot to assimilate and decide on all the information available.