As to why logical positivism is not successful in fields in science and engineering -- I would say that in general it is not possible to say that "any two observably equivalent theories are one and the same theory"
In particular, outside of science and engineering, it not possible to get a sufficient precision on what is "observably equivalent".
Actually, sufficient precision is usually not possible within science and engineering either.
As to why logical positivism is not successful in fields in science and engineering -- I would say that in general it is not possible to say that "any two observably equivalent theories are one and the same theory"
In particular, outside of science and engineering, it not possible to get a sufficient precision on what is "observably equivalent".
Actually, sufficient precision is usually not possible within science and engineering either.