Because a programme needs long enough to be judged fairly. Setup, invitations and the first campaigns all land in the first few weeks; three months gives your participants a full quarter of earning and redeeming, so the review at the end looks at real behaviour rather than a launch spike.
The minimum initial term on your first Statement of Work is three months, on every tier.
After that initial term, it runs month to month.
Two things worth knowing. The minimum applies to your first Statement of Work only — if you later add a second programme, that Statement of Work carries no default minimum term of its own. And where your Statement of Work includes an early-cancellation right, it applies during the initial term too, on 14 days’ written notice.
Short programmes underperform. A six-week trial would tell you little except that six weeks is not long enough.