In a Ketubah, the Rabbis instituted that a man should obligate himself to pay his wife, should he chose to divorce her (or he dies). If she had never been married before, he obligates himself to pay 200 zuz. If she was previously married, he obligates himself to pay 100 zuz. (See Rambam Hilchot Ishut 10:7) [That is the minimum, he could offer more if he wants]
The Bartenura on Mishnayot Peah 8:8, says that 200 zuz is enough to support someone for one year.
If that's the reason the husband promises 200 zuz, why is he only obligated to pay 100 zuz to a widow/divorcee? Why did the Rabbis feel it was enough to support her for only half a year?
If that's not the reason, what is the reason for the discrepancy between a previously unmarried woman, and a previously married woman?