Why were Dalton's ideas about atoms considered a scientific theory whereas the ideas of ancient Greek philosophers were not?
Because what Dalton proposed and theorized was testable....
And for a fact we don't know what old Democritus proposed. He might have had a sophisticated understanding of chemistry and atomic theory...he certainly did not communicate it.
On the other hand, Dalton's atomic theory was made on the basis of evidence and data....and further, the experiments that Dalton performed could be performed and considered and modified by other researchers....his competitors and rivals, as well as his collaborators.
And thus we get the modern science of chemistry, developed after a MERE 200-300 years....
As regards scientific theories, one of their most important characteristic properties, is their