One reason might be that you may want different stuff in the Rockies than you did in the East and the plains. You may want colder weather clothing, heavier sleeping bag, or other stuff that you may prefer not to carry in the East. If I were doing another Trans America and was going E-W I would be likely to leave my colder weather stuff home and have it sent to me when I reached the mountains. Going East we sent a lot of stuff home when we left the Rockies and had a few different items sent to us. This was mostly getting rid of warmer clothing.

If headed East... you may decide you can splurge on a heavy item or items once you are out of the mountains. For example when riding the TA from W-E my daughter wanted to read a big heavy fluid dynamics text book in preparation for grad school. Carrying it in the mountains was out of the question, but on the plains it was worth it to her. So it was mailed to her when out of the Rockies and back home in time for the Appalachians.

Also you may want some hard to find locally type items. I'm not generally an advocate of carrying a lot of food, but we were given a lot of good stuff by NOLS and broke it down into 5 lots shipping 4 ahead to different post offices. The stuff we shipped ahead was stuff like dried hummus, freeze dried veggies, freeze dried apple cider, freeze dried refried beans, and dried red beans and rice. It was light enough that we didn't mind carrying some of it, but not light enough to carry it all at once.