Presumably (am not an expert in this!) because the barrier to entry in a field of knowledge was lower in the past. As in, at one point you could read all there was to know about a subject.
Also, there was more 'low hanging fruit' in the past. No one is impressed now if you reinvent the calculus. So it used to be easier to have major discoveries in multiple fields.
Finally, the whole idea of different areas of knowledge is relatively modern. The 'sciences' used to include astrology, for example. Even the distinction between biblical studies and observation of the natural world was vague at some point.
Also, there was more 'low hanging fruit' in the past. No one is impressed now if you reinvent the calculus. So it used to be easier to have major discoveries in multiple fields.
Finally, the whole idea of different areas of knowledge is relatively modern. The 'sciences' used to include astrology, for example. Even the distinction between biblical studies and observation of the natural world was vague at some point.