Why would the Arkansas Territory be allowed to have slavery after 1820?

Though ostensibly free-soil, the new state had a constitution that permitted indentured servitude and a limited form of slavery.  Taylor also had antislavery credentials since in February 1819, he had proposed a similar slave restriction for Arkansas Territory in the House, which was defeated 89–87.

Explanation:

bismarck believed in using force

is the correct answer!