Because Facism is evil.
And Socialism, in its purest form, is about equality.
All political systems have countless examples in history of those in power using said power to abuse [or jail, or torture, or disappear, or exterminate] the populace. Just as a presidency or monarchy is only as good as the president or monarch, any economic or political system is only as good as those allowed to remain in power are.
Socialism is not 'about equality', or at least, that's a vast oversimplification. Socialism is an economic/political system in which either the community or gov't owns the means of production and distribution of goods and, ultimately, there is no private property. Is that a better system than capitalism? Well, maybe [and I'm no fan of capitalism, though it's certainly an effective economic system for certain time periods]? But it's not inherently good, nor is it necessarily 'about equality' since if history is any guide you can be sure those in power will still find a way to be the 'haves'.
Grant the socialist government too much power, and it should be easy to see how a system in which the government owns everything can go sour real, real quick. And it's a quick shift from 'community owned' to 'government owned'.
More importantly, we need to question -how- a new economic/political system falls into place. For example, how does a society of capitalism shift to socialism? If individuals currently own everything... how exactly does the gov't take control? The short answer is: force. They take everything. Your family business that's five generations old? It's now the government's. Or, how are political dissidents handled? That often has little to do with the economic system in place, and everything to do with the will of those in power.
The old adages that 'power corrupts' and 'money is the root of all evil' are both a bit oversimplified as well, but in general, should be remembered in this case. All political systems and all economic systems can be corrupted. That certainly includes democracy and capitalism, or what most consider those to be nowadays anyway. Socialism certainly has it's good points, but don't make the mistake of oversimplifying and think it's all roses and 'equality'. It's not that simple.