Why are there no stocks quoted on the OTC Bulletin Board (OTCBB)?

Update:

In September 2020, FINRA proposed to shut down the OTCBB due to lack of activity (SR-FINRA-2020-031 [PDF]). While the OTCBB still exists, no broker-dealer has used it in the past year. Excerpts from the proposal:

... the level of quotation activity occurring on the OTCBB has continued to decline over the past several years and is now nonexistent. In fact, as of the date of this filing, the OTCBB does not display or widely disseminate quotation information on any OTC Equity Securities.

Of the 352,698 average trades per day reported in all OTC Equity Securities in August 2020 (with a total of 7,406,664 trades reported in all OTC Equity Securities for the month), none were related to quotation activity on the OTCBB. No member firms have quoted on the OTCBB since October 29, 2019.

In fact, as of the date of this filing, the OTCBB does not display or widely disseminate quotation information on any OTC Equity Securities.

Looks like the OTCBB will be gone soon.

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Original answer:

I am still unfamiliar with OTCBB and OTC securities in general, but I found some information by digging around FINRA's website. I'd appreciate a better answer from someone more experienced. Here's what I found:

Looks like there are no longer any securities trading on the OTCBB. FINRA's Market Statictics shows that the trade volumes on OTCBB have declined significantly in the last 5 years:

Compare to "Other OTC", which has far higher trade volumes:

It appears that there has been no trades on OTCBB in the last six months (as of July 2020), with the last trades occurring in November 2019:

Apparently, for most (all?) of 2019 there were only two securities traded on OTCBB: "Vantage Drilling International Stapled Unit Consisting Ord Sh & 1%/12% Step UP Sr Secd (Third Lien Conv Nt Due 2030)(Caymay Island)" (VTGGF), and "Signature Bank Georgia Sandy Springs Common Stock" (SGBG). VTGGF appears to be the only security that traded in October 2019 and November 2019.

From what I can tell, market participants seem to have left OTCBB and moved to other Alternative Trading Systems (ATS) such as OTC Markets' OTC Link ATS (whose trade volume is included in FINRA's "Other OTC" statistics).