Why are communism and fascism considered at opposite ends of the political s pectrum?

The reason is because the political systems of government can be looked at based on power and wealth distribution and the populace of a nation and measured by the wealth gap of citizens. The spectrum can be looked at like this.

Fascism - power = power (force/military/police state) (Nazi Germany)

Capitalism - money = power (rich rule) (really what the US is today)

Republic - majority = power, however minority rights are protected through a Bill of Rights all citizens have in order to protect them from an oppressive minority (religious freedom for example), and wealth distribution is not strongly governed however there is a basic infrastructure in place to allow motivated citizens to Excel. (What the US Constitution attempted to establish)

Socialism - majority = power, however the rights of citizens are more progressively controlled through the system in order to ensure that all citizens are afforded equal opportunity regardless of how wealthy they are born (infrastructure, education including college, medicine, living wage for students, etc... is paid for by collective taxation). (France, Canada, Holland, Sweden, etc..etc.. most civilized democracies)

Marxism - ruling class = power, wealth is distributed in order to have society work as a single unit, greatly reducing the wealth gap, information is controlled.

Communism - ruling class = power, however the government's job is to ensure that wealth distribution gives equality to all citizens regardless of skill, all citizens are equal (in "ideal" communism).

In this respect, fascism is force of the ruling class, and the bulk of the nations wealth is in the hands of a very small % of citizens. On the opposite end of the spectrum communism's goal is to eliminate any wealth gap (or even a need for money, food is often evenly rationed, housing is provided by the government, citizens are assigned jobs, etc...). There is also the concept of individualism at the fascist/capitalist end where a citizen born into privilege will simply Excel, and a poor citizen will die in a gutter without support. The systems range from selfish to selfless in respect to individualism.