Rational ignorance

Rational ignorance occurs when a decision-maker chooses not to gain more information because the costs of doing so would likely outweigh the benefits.

Examples
If you were to visit an unfamiliar town that you are unlikely to visit again, you may opt to simply buy petrol at the first station you see rather than drive around looking for the best deal, because the few pennies' difference in price would not be worth the time and petrol expended looking for the best rate.

Likewise, voters will sometimes not learn everything there is to know about all the issues and candidates because the odds of that single vote swinging the result are perceived as so small as to make it not worth the time.