Relativity also needed data that we only could produce and end of the 1800s and beginning of the 1900s

In this case, the bottleneck has been a deliberate refusal to work on the underpinnings of QM. The Copenhagen "Interpretation" refuses to consider what a measurement is, and creates an artificial separation between a quantum system and the system taking the measurement. For sixty years, adherents were told to "shut up and calculate" rather than ask questions like: Isn't the apparatus taking the measurement also a quantum system? Why should there be two sets of rules, one when a measurement is made and one when a measurement does not occur? What is it about measurements that cause this? What mechanism is going on in the universe that causes it to appear to function in the classical, emergent way, the world of Newton and Relativity, rather than in superpositions and spooky action at a distance?

Part of the reason little progress has been made is that it was considered career-ending in physics to work on those questions, because some of them dive into philosophy (What is knowledge? What is real? What is emergent?) Throughout the latter half of the twentieth century graduate students were counseled away from any work related to the field. Oppenheimer famously organized the shunning of Bohm for daring to publish (at Einstein's encouragement) on the subject. Hugh Everett was driven out of academia altogether.

Hard problems can be solved when people actually work on them. Often the problem is other people preventing the work. Thankfully this attitude seems to be easing thanks in part to the efforts of people like Sean Carroll and Lee Smolin.