The short answer: juiced ball. Power numbers were waaay up in 1987 compared with other years in the 1982-92 era.
The more nuanced answer is that the best players in 1987 -- ED, Raines, and Jack Clark all missed significant time due to injuries ( and contract issues in Raines' case). That complicated the decision. Ozzie Smith was great that year too, but he hit 49 fewer HRs than Dawson.
In the 80s, baseball writers had no idea of the relative value of defense, base running, and power.