Many of the European powers had issues on their plate as it were in the middle of the 19th century. Although the Ansei Treaties were extremely unfavourable, do not forget that the Japanese were more then willing to fight, and as Jon Halliday notes "the organized resistance of the Japanese ruling class. It had a long and uninterrupted tradition of rule behind it... The Japanese regime was not completely successful in keeping the Western imperialists away, but it certainly made penetration of Japan appear a daunting, and perhaps unrewarding prospect."

Moreover, resources were stretched due to many other global events: Halliday states that "The Opium War, followed shortly by the Taiping rebellion, a had a stimulating effect on the Japanese. The idea of resistance to imperialist pressures was further strengthened by the great revolt in India." Russia do not forget made forays and clashed as well, but was more looking at a land grab on the mainland. The US was more interested in Japan as a strategic interest with regards to Chinese access.

Halliday also notes that "The Opium war had two wider effects which connect closely with why Japan was not turned into a colony. First, the grab for China tied up most of Britain's energies in the area over the ensuing decades. Second, the forcible opening of China for foreign trade and the concomitant weakening of the Manchu regime put penetration of China right at the top of most imperialists' list of priorities."

Don't forget that the European continent itself was not a peaceful place at this time. Resources had to be spent on the Crimean War, and later on the Franco Prussian war. American pressure eased due to their civil war as well.

The thing that interests me the most I found was this insight by Halliday - he states "that due to the effects of the Satsuma and Choshu [which closed the Shimnoseki Straits] were originally both very anti Bakufu but also along the anti foreigner Han both were defeated by the Western powers, with may have caused certain samurai leaders to advocate a policy of allying with the West against the Bakufu. The Western imperialists, led by Britain; tacitly support these Han, and persuaded the foreign legations to remain neutral."

It was not just the West that hindered themselves though - The Chosu defeated two Bakufu punitive expeditions, because they were more organized. Japan also had the Emperor actually step in at some points calling for the expulsion of the West, and only eventually consenting t the Ansei treaties as he was persuaded there was no other choice.

Source Halliday - the Meiji state

Further reading The Meiji restoration, W.G. Beasley