Why didn't the united states fully investigate the u. S. S. Maine explosion before going to war with spain?

2 major reasons: support for intervention in the Cuban war for independence and media exaggeration of the explosion.

Many Americans had been already been pushing for the US to get involved in the Cuban revolution and their fight against the Spanish. The Spanish had been violating human rights and been using concentration camps to imprison Cubans. In addition there had been pushes in the past to expand US land to Cuba. Some wanted Cuba as a colony, others as an economic trade partner.

The media also played a large role in the lack of an investigation. The major newspapers used the technique of "yellow journalism" which was an exaggerated form of creating media. The headlines immediately blamed Spain for the explosion. This convinced the American people that Spain was to blame. This puts the government in a difficult position of needing to defend the country. Being there was already some support it made it easy to continue with the information they had and was being reported.