Because it's not just cyclical major depression (and there's no "just" to major depression either, but that's another rant). The diagnostic criteria for bipolar is that you have had at least one extended episode of mania or hypomania alongside a depressive episode, so you're not going to have the diagnosis unless you've been manic or hypomanic at some point. Although it may not be a major or dominant part of someone's presentation, mania or hypomania is nevertheless there and it effects what treatments are appropriate. Certain SSRI's may trigger off hypomania or mania in a bipolar patient, or simply be utterly ineffective, whereas in someone with cyclical major depression their response would be different. I'm bipolar II and spend roughly 1/4 to 1/3rd of my life meeting the criteria for major depression but very rarely have hypomanic episodes, and this was the case even before I was on lithium. Indeed I've gone for years in between major hypomanic episodes. But hoo boy! When they hit there is no mistaking them.
The other thing is that unlike with depression where you have to squint really hard to see much of an effect from most treatments, there are reasonably effective treatments for mania and hypomania, including good old lithium. So if you're being treated, while you might still suffer from regular bouts of depression, you'll likely have much reduced or even eliminated hypomanic or manic symptoms. So it's not unusual to see manic depressives who are treated showing very few manic symptoms for years on end.
