why did manic depression become bipolar disorder

The current version (DSM-5) lists the following subtypes of bipolar disorder with the following diagnostic criteria:

Bipolar I Disorder
at least one manic episode and one or more major depression episode
equally common in men and women, with the first episode in men usually being mania, and the first episode in women typically being major depression.

Bipolar II Disorder
major depression
instead of full-on mania, they experience hypomania: high energy, impulsiveness, and excitability, but less severe as full-fledged mania.
more common in women than men

Cyclothymic Disorder
less severe mood swings
episodes shifting from hypomania to mild depression
rapid changes in mood — with four or more episodes of major depression, mania, hypomania, or mixed symptoms within a year.
may have more than one episode in a week or even within one day
more common in people who have their first episode at a younger age
affects more women than men