Why did Mendel prevent his plants from self-pollinating?

to control the crossing of the traits

in order to insure that each plant's offspring would remain identical to the parents

to maintain a pure strain

so they could not produce offspring

The answer is to control the crossing of the traits.

The idea of preventing plants from self-pollinating is to control the crossing of the traits. If plants were self-pollinating, he could not know if the traits are inherited from the same plant or from the other. By preventing this, he could cross parents with known phenotype and that way learn about crossing process and inheriting process.