Looks like David Hartley’s Observations had a huge cult following for quite some time among Unitarians in England and the USA but when it disappeared, it really disappeared.
“By the late 1800s Priestleian Unitarianism, as a tradition of philosophical practice, had died out; academic philosophers, especially those trained in the Scottish school of common sense (the official philosophy taught, for example, by the Unitarians who ran Harvard), knew that the Observations rested on a fundamental mistake; and association psychologists preserved a memory of this precursor of their science. Formal interest in Hartley’s Observations, as a work to be read and pondered, discussed in lectures, written about in articles and books, had collapsed. The last serious engagement with the Observations appears in William James’s Principles of Psychology (1890, 1:553), and even there a casual reader could easily miss it.
In the twentieth century, Hartley’s Observations remained outside the philosophical curriculum: philosophers-in-training could follow the well-traveled route from the “British empiricists” to Kant without turning aside to pick it up. And even if a person had wanted to, doing so was difficult, for after the editions of 1749, 1775, 1791, 1810, and 1834, and translations into French (1755 and 1802), German (1772), and Italian (1809), no further editions were published. Today, paperback reprints of earlier editions are available, but there is no modern critical edition.”