IAAF officials said in a statement that awarding Eugene the 2021 championships in a summary fashion was justified because of "a unique strategic opportunity" to host the World Championships for the first time in the United States. This had been a long-held goal of outgoing IAAF President Lamine Diack.
"Although this decision departs from the usual procedure, I am delighted that my Council colleagues understood the enormous opportunity presented to us to access a key market and have taken a decision in the interest of the global development of our sport," Diack said.
Hayward Field's pedigree as a venue for track meets with enthusiastic and knowledgeable fans is not in question. The stadium hosted the 2012 Olympic Trials, and will host the 2015 national championships and 2016 Trials. The Prefontaine Classic, a Diamond League meet, is held every year in Eugene, and the NCAA Championships are often held in Eugene (they will take place there this year).
Nonetheless, the IAAF decision is likely to be controversial. Hayward Field's seating capacity is much smaller than the usual mega-stadiums used for world championships. According to Wikipedia, the regular capacity of the stadium is 10,500 but is expandable to 21,000. The facility will be expanded further to host the world championships, but will still be much smaller than a facility like National Stadium in Beijing, which was used for the 2008 Olympics and will be used for this summer's World Championships. It has a capacity of 80,000.
Moreover, Eugene is a small city by global standards with a population of about 160,000, and lacks a major airport.
The exact dates of the 2021 championships were not released, but the meet is usually held in August.