Why those buses?

On the Japan-U.S. Forum hosted by the National Bureau of Asian Research (NBR), William Stonehill speculated,

"...killing the JNR has also allowed some entirely non-bus companies to tentatively stick their
toes in the water for local bus routes too. Bunkyo Ku in Tokyo is running a bus called the "B-bus" which only runs in Bunkyo ku and appears to be unaffiliated with Toei buses run by the city. Nobody has rose up and slapped them down, so maybe the next step will be privately operated buses running between stations after the JR shuts down for the night."

I don't think that is the real reason for these buses, and the non-bus companies in this case are the local cities and wards. I believe these B-bus like operations, Community Buses I think they are called, using mini-buses, are something new across Japan to serve the elderly population. In my area, Mitaka, Musashino, and Koganei cities we have several of these small buses. They also operate in Suginami-ku where Toei bus operates. Ours is named the "CoCo bus." They are only 100 yen to ride, senior passes are not valid, and the circular routes originate and end at a single train station after passing through small neighborhood streets. This is the type of bus that also is used to shuttle visitors to the Ghibli museum from Mitaka station.

Koganei City's "CoCo Bus"

Surely, the cities (i.e. ku and shi level governments) must be subsidizing these services. As I noted above, these small buses compliment the service provided by the longer-range, point to point bus service provided by Toei Bus, Odakyu Bus, and Keio Bus, each with a monopoly over specific geographic areas. At 100 yen per person, there are not nearly enough passengers to pay the drivers salary, even if it were at a minimum wage.

The buses are actually managed by Odakyu and Keio bus, the local operators, respectively. That is, the drivers are provided by and the buses maintained by the local bus operator. I see them parked at the local depot at night. It seems to me that local cities are having to provide these handicap accessible services (only running from 9am to 7pm, inconvenient for commuters) to get the elderly out of their homes.