What was the reason for the decline in the rate of Delta launches?

During the first half of the 1980s, the rate of Delta launches drastically declined due to the Space Shuttle taking a large part of its missions. Orders for the booster slowed to a trickle and McDonnell-Douglas came close to completely shutting down production. 1985 was the first year since the Delta family's introduction in 1960 that no launches took place, but the Challenger Disaster brought about a renewed need for disposable launch vehicles and orders soon picked up again. While only a handful of Delta launches took place in 1986–88, nine flew in 1989 and the 1990s would see a busy schedule most years except 1994-1995 (seven launches during those two years).

Delta 178 was largely the result of poor quality control due to plans for eventually phasing out ELVs in favor of the space shuttle - most of the engineers who worked on the program had left and one senior McDonnell-Douglass engineer remarked that 178 had "the worst quality control I ever saw on a Delta vehicle". Because it was assumed that the Delta would soon be retired from use, complaints about the poor assembly quality went unheard. After the accident, a thorough investigation of wiring systems in various ELV families was undertaken, which discovered that, among other things, the Delta had thinner wiring insulation than Titan, Atlas, and other launch vehicles. Moreover, Teflon insulation had the unique ability of melting and re-fusing when heated by electrical arcs. Eventually however, a short occurred that overpowered the Teflon's "self healing" ability.[2][3]
The rate of Delta launches declined due to the Space Shuttle taking a large part of its missions.