What is really crippling the progress of string theory?

The fact that it's not a theory, and all the work on it has already given up on following the scientific method in order to arrive at a real theory. The entire matter is really a coup inside the theoretical physics industry, where a bunch of worthless mathematicians took the lead, and all they do is sit around totally absorbed in their math, oblivious of the previous need for, oh gee I dunno, RUNNING EXPERIMENTS to test hypotheses in order to ultimately support a real theory.

Personally I suspect that the entire physics industry just matured and found itself looking for the easy way out. Physicists were unable to find a link between relativity and quantum mechanics, despite decades of trying. So they were looking like fools. String 'theory' gave them the opportunity to work pretty much forever on physics without looking like fools to the common man for their inability to actually come up with a workable theory. Job security, forever. The new paradigm in physics is that you can sit around and play math games and never actually spark any experiments or verification process of any sort... and you still get paid your salary and your grants keep getting funded. *But there are no results.*

Hell, that's the dream of K-12 unionized educators in public systems, and they achieved that dream a long time ago. They get paid for no results, or results that are so piss poor that rational socio-economics would have fired all those people and blacklisted them from the industry due to their massive incompetence.

Anyway, to day, string 'theory' has produced no predictions, and in fact is unable to produce them, be design. It's not a theory; it's a way for about 1500 physicists worldwide to collect paychecks even though they don't actually produce. We've about the same number of string 'theorists' today as we did in all of physics when Einstein was working in the Swiss patent office. The industry became bloated.

There's a very difficult thing about Dark Energy that makes it hard to detect at other than supergalactic scales. The speed of the expansion of the universe over Human scales is very, very small. Each linear meter of space is expanding at roughly 10^-18 m/s. Protons wobble in their nuclei much more fiercely than that speed. That speed equates to roughly a lightyear growing longer by a few centimeters per second; note that a lightyear is about 10¹⁸ centimeters long.

Those sort of speeds are ridiculously smaller than our instruments can detect, and they have no measurable effect on matter at those scales. Over the scale of our galaxy, the expansion speed rises to about a kilometer per second, and that sort of speed is still swamped by stellar movement which is on the order of 100 km/s.

It's only over galactic scales that the speed starts to appear in the data on frequency shifting of light.

Hence, we look for evidence of DE at the scales where it appears: Supergalactic scale. And that's all we've been able to know about it. Further examinations are taking place today that involve very detailed observations of long-range photons, since they've crossed billions of lightyears. We don't have reliable conclusions yet from such data. Give it a few more years.