Because the 'relationship' with a Performance Rights Organization (PRO) like BMI is not 'active', with lots of 'cuts' and 'releases' to market, instead being 'passive' with one or two compositions getting to market but not getting enough plays in revenue-generating venues, within a short time scale, the 'relationship' tends to go dormant.
The 'Registrant' who 'Registers' a Song Title, the names of the 'authors/composers', copyright owners, and Publishing Company name, tends to forget about the 'membership' and 'relationship', and time passes. People move and don't update their contact information. Phone numbers change. 1976? Land line phones disappear. Cell phones appear, and go through evolutions. Addresses change. The music biz goes into chaos and Publishers go out of business.
If the Song got enough plays to earn any Songwriting Royalties and Publishing Royalties the PRO would have paid them directly to the Songwriter of record and the Publisher of record, if they could contact them. If the check came back, undeliverable, they simply held the money. If either had 'disappeared', the contact no longer valid, they don't do much searching to find you. They have lots of people who do have money coming who have not updated their contact information and therefore are not 'contactable' in the passivity. By 'lots' I mean the overall quantity, not necessarily your one Song. Although...a few hundred Euros might be a nice shot in the bank account in 2020.
Have you checked with BMI?
Did you start your own Publishing Company and is that the Publisher Registered with BMI?
Or did you sign a Publishing Contract with an established Publisher? Is that Publishing Company still in business?
Has contact information changed since 1976?

There will always be another song to be written. Someone will write it. Why not you? www.garyeandrews.com