Well if you want to compare to why C++ has multiple inheritance, you need to first explain which mainstream languages in the mid-1980s had multiple inheritance so we can contrast.
Like algebraic data types, multiple inheritance wasn't novel, but what were the big famous languages from that time which showed this was a must have for Bjarne's language?
That's not the comparison I'm making. It's possible for a proposed language to add something despite not being popular at the time, but that's not the default. I'm suggesting that if ADTs were not already popular at the time, it would be the unsurprising default for them to not be in a new language. In today's language landscape, by contrast, it would be surprising and disappointing for a new language to not have either ADTs or a specific alternative they propose for solving the same problem.
But the actual answer is revealing. Bjarne implemented multiple inheritance in C++ because it was easy. It's a New Jersey language. The priority is simplicity of implementation.
It's not about what was popular, or that Stroustrup was at Cambridge not Edinburgh (all the exciting Programming Language theory stuff in that era like ML was from Edinburgh as you perhaps know). If ADTs were easy, C++ would have ADTs.
Like algebraic data types, multiple inheritance wasn't novel, but what were the big famous languages from that time which showed this was a must have for Bjarne's language?