This happens because there is a general rule for the pronunciation of Spanish words and where the stress goes. An accent over a vowel means that vowel is stressed instead of the one that normally would be if there were no accent.
Example, words ending with an 'n' or an 's' or any vowel are usually stressed on the second-last syllable. But región has the stress on the O, so it needs the accent because otherwise the E would be stressed.
When it becomes regiones, the o is now the second-last syllable and thus is already stressed without need for accent. Get it?