Why have white sprinters stagnated?

Even if genetic differences contribute to black dominance in short sprints, the question of why white sprinters have stopped getting better remains:

Today’s few elite White sprinters can run no faster than their predecessors from the 1970s, despite improved equipment, support, and facilities (George, 1994). Proposed racial physiological differences would not adequately explain White sprinting stagnation over a quarter of a century. Proponents of biological determinism might stress that whilst racial athletic differences are small, split seconds can separate champions and also-rans (Entine, 2000). However, the influence of stereotypes could also account for performance differentials, with Whites effectively defeated at the starting-line, by inflated impressions of Black rivals. For White sprinters fear of failure, and over arousal could be triggered by negative stereotypes, whilst Black sprinters may be more relaxed, and confident, due to positive stereotypes. It certainly seems that contemporary sprinting is more important in Black subculture (George, 1994), and few Whites choose to participate, perhaps because of perceptions of inferiority. Coaches may be significant agents in shaping attitudes and channelling Black or White athletes into or away from sprinting due to stereotypical assumptions.
[David Turner and Ian Jones. False Start? UK Sprint Coaches and Black/White Stereotypes.]

Here is the abstract of the paper cited above:

Over the past 30 years almost all world-class US sprinters have been black. There were also many fast black sprinters in the USA before the 1960s, but in addition there were a considerable number of world-class white sprinters. In fact, during the 1940s and 1950s the fastest men were white. This was not the case during the 1930s, when the best male sprinters were black. This essay discusses the phenomenon and attempts to give reasons for it. Sociological explanations seem considerably more plausible than physical characteristics based on perceived racial differences.
[George, J. (1994) The virtual disappearance of the white male sprinter in the United States: A speculative essay. Sociology of Sport Journal, 11, 1, 70-78.]

Black dominance in sprinting coincides with the advent of widespread anabolic steroid use. Perhaps this is entirely coincidental. Regardless, when boosters of black athleticism note that no white has run 100 meters in less than 10 seconds, one can note that neither did any black in the pre-steroid era or without Western coaching.