One reason for the one year difference between irusin/kiddushin and nisu'in was to give the groom time to find a trade and prepare a home so that he could support his family, or to continue his Torah learning.
One reason for the change to how it's done nowadays(with nisu'in right after irusin under the chuppa) was because there were a few cases where the bride failed to remain faithful during the one-year break. Another reason was because of the poverty that prevailed in most Jewish communities and they couldn't afford to undertake two ceremonies.
These are some explanations I heard that made sense to me when studying the sugya.
See here for more info:http://www.chaburas.org/erusin.html and http://www.chaburas.org/erusin2.html.