Why can’t the newest high-bypass turbofans run at their EPR, temperature, and RPM limits simultaneously at sea level?
What makes it impossible for very new turbofan designs to reach all three redlines simultaneously at sea level?


The implication in your question is that GE9X was not able to reach all three redlines simultaneously but that previous engines (e.g. GE90, GEnx, LEAP, etc) could.  That is incorrect.  Those engines could not reach triple redline either. At least not, type design unmodified.  The engines that were used to demonstrate compliance to FAR 33.87 for those engine programs were modified from their type design in order to be able to reach triple redline. Very heavily modified. 

To understand why, let's talk a little about engine performance.  The three variables of N1, N2, and EGT are controlled by several conditions: the throttle setting, the ambient temperature, the ambient pressure (i.e. altitude), the forward velocity, and the deterioration of the engine (e.g. how worn out the seals are).  Now, if we are doing this test on a test stand (as opposed to flying it), then we really only have control over one variable: the throttle.  Everything else just is what it is. It shouldn't be too hard to see that you can't control 3 variables precisely with just one knob (E.g. imagine in your bathtub or shower you try to control both the water temperature and water flow rate with just one knob. You can't do it.  To control 2 variables, you need 2 knobs).

More specifically, considering sea-level static conditions, if the ambient temperature is cold, then the air is dense, so the fan pumps a whole lot of mass with every revolution and makes a lot of thrust.  You have to burn a lot of fuel to do that.  So you'll hit EGT limit first before you get anywhere close to N1 redline.  If the ambient temperature is hot, then the fan doesn't pump as much per revolution, so speed needs to be really high in order to make any kind of thrust.  You'll hit N1 redline before you hit EGT redline. You can presume that there should be one magic temperature where you could hit both of those at the same time, but if you just sit around and wait until the weather hits the exact right temperature you wanted and stays there for 150 hours (the length of the test), you'll be waiting a long time.  And even then, you still won't get to N2 redline at the same time.

Ok, so how would you run a triple redline test then, given that we can't control ambient temperature? Well first you'll need to way to vary the thrust the fan produces.  E.g. you could use a variable fan nozzle.  On a cold day, you open up the fan exit area so you can get to higher N1 speeds with less thrust, and on a hot day you throttle it down.  There are other options but this is easiest to explain.  Then you need to control N2.  The easiest way here is to open or close the variable stator vanes.  This has the effect of making the HPC do more or less work per revolution, so you can tune in the exact speed you want.

Ok, so we added a variable fan nozzle and a different VSV schedule, and we are at triple-redline.  Problem solved right?  Wrong.  Jet engines are extremely complicated devices with all kinds of interdependent requirements.  If you change one thing, you end up affecting other stuff.  Changing the VSV schedule might introduce aeromechanics issues onto the HPC blades, or change the cooling flows to different portions of the turbine.  So then you have to make other modifications to the engine, like changing the sizes of all of the different cooling pipes so you get back to the original flow rates you wanted.  But then, those changes also have other effects, which snowballs.

Historically (I mean like 1970s 1980s) the engine companies could make a few small changes and hit triple redline.  As the engines became more and more complex, more and more changes were required. At some point, the list of changes required to make the engine hit triple redline is so big that the thing you are testing is a crazy chimera of an engine that has only a faint resemblance to the actual type design.  At that point, running a triple redline test no longer makes sense.  That's what happened with 9X. Therefore, GE found an alternative way to satisfy the endurance test requirement.