There was also a lot of relatively low-hanging fruit in most fields, because we basically didn't have the technology before, or simply didn't bother to look.
There's a ton of low hanging fruit now - more than ever. Every question answered raises multiple new questions. If there is a lack of opportunity to answer interesting questions, it is certainly not because the questions aren't there.
But someone needed to realise it's a low-hanging fruit in the first place. By the end of the XIX century the general agreement was that physics was pretty much done, we are now just polishing the details.