Simply because electioneering does not work. History is littered with examples of radicals being voted into office only to become as, or even more, conservative than the politicians they replaced.
As we have discussed previously (see section B.2 and related sections) any government is under pressure from two sources of power, the state bureaucracy and big business. This ensures that any attempts at social change would be undermined and made hollow by vested interests, assuming they even reached that level of discussion to begin with (the de-radicalising effects of electioneering is discussed below in section J.2.6). Here we will highlight the power of vested interests within democratic government.
In section B.2 we only discussed the general nature of the state and what its role within society is (i.e. "the preservation of the economic 'status quo,' the protection of the economic privileges of the ruling class," in the words of Luigi Galleani). However, as the effectiveness of the vote to secure change is now the topic we will have to discuss how and why the state and capital restricts and controls political action.
Taking capital to begin with, if we assume that a relatively reformist government was elected it would soon find itself facing various economic pressures. Either capital would disinvest, so forcing the government to back down in the face of economic collapse, or the government in question would control capital leaving the country and so would soon be isolated from new investment and its currency would become worthless. Either way, the economy would be severely damaged and the promised "reforms" would be dead letters. In addition, this economic failure would soon result in popular revolt which in turn would lead to a more authoritarian state as "democracy" was protected from the people.
<snip>
http://www.infoshop.org/faq/secJ2.html