Had a tubulovillous adenoma w/hgd removed via right-hemicoloectomy. Why does a repeat colonoscopy have to be done in a year if polyps grow slow?

One year is correct. Colon cancer develops from a polyp with high grade dysplasia. A history of dysplastic polyps puts you at high risk of forming new polyps and the pace of growth is uncertain, especially in a young age group. There are clear guidelines put out by the american cancer society on the suggested followup for colon polyps. If your next colonscopy shows no polyps, they will extend the time interval.
It the dysplasia. Think of your disease as representing a field defect. Other areas of the colon may have the same defect and allow for growth of other areas of polyps or high grade dysplasia (a precursor to developing cancer). Follow through to keep yourself safe. Always better safe then...