Because "le cose" is plural = the things. The singular is "la cosa" = "the thing". Adjectives like importante have to agree with the noun they modify in terms of gender and number. Importante is singular (sometimes nouns and adjectives end in "e"). If it ends in "e" in the singular, it ends in "i" in the plural. I think your confusion arises because you think the adjective should have the same ending as the noun. Unfortunately, it doesn't work that way!