There is also the fact that Kosminski's personality profile fits that of a serial killer, he had a deep hatred of women, and he was a butcher (and had a knife that matched the cuts). Being a Polish Jew, Kosminski was a likely match for the person who left the 'Jewes' graffiti. This being the case, and with the already prevalent anger against Jews and immigrants in the wake of the murder and the news of the graffiti, it seems likely that the police actually knew they had their man, but did not want to prosecute him publicly for fear of starting an anti-Jewish progrom in London. So they locked him away and made certain that he could never get out.
Shortly after the anniversary of the murders, there was a television special where several experts were asked to weigh in on who the killer was. The most qualified person on the panel, a woman who worked on investigations of serial murderers, said that Kosminski was the obvious suspect, but the audience went with the Queen's Doctor theory because of a TV special that offered that theory--despite the fact that at the time of the murders, said Doctor had already suffered a stroke, and had lost the use of one of his hands. Conspiracy theories always favor the most powerful agencies for events of broad prominence; this is why large government conspiracies are always favored over individual (Lee Harvey Oswald) or small group (Al Quaeda) actors.