Besides the size and weight, it has to do with the internal action mechanism. The Redhawk and SRH have a heavy double-action trigger and yet they still have a slow trigger reset. The wikipedia article for the Redhawk explains, "The revolvers also used a single spring for both the hammer and the trigger, and this meant that the force required to pull the trigger was higher than similar offerings from other manufacturers, and there was no way to adjust or correct this as it was inherent in the single-spring design." A rapid string of fire is not reliably practical. It's too easy to over-run the trigger. Maybe that has nothing to do with "real world combat" but since nobody seems to be able to define what's actually needed for "combat" we just have popular consensus to go by. Will a Redhawk stop a mugger? Probably. But it is not a reasonable option for IPSC, IDPA, or USPSA, which are more or less intended to test practical shooting. Certainly, it's not combat, but it is intended to have a measure of relevance to gunfighting. We can also look at police-issue. Redhawks were never intended for and were never used for duty.
The Super GP-100 gives us a revelation. What Ruger did was to put the GP-100 action mechanism into a Redhawk-size frame:
The Super GP100 is only available in 8-shot 357, but the same action is available in 6 and 7 shot GP100's in 327, 357, 40, 10mm, 44 Special etc. If Ruger offered the Super GP100 in 44 Magnum or 45LC/ACP, then they would have something really comparable to the Model 29/629 and 25/625 etc. But the market for "combat" 44's and 45's is limited. People shooting combat-style competition are mostly using 357 and 9mm and people carrying are mostly carrying .38/357 or a pistol (9mm etc.).
Again, I'm not trying to sell a cartridge. I just want you to be aware of the difference with a Redhawk action. Good actions are available in 44 or 45, but I'd recommend a 44 Special GP100 or a S&W Magnum or Long Colt, ACP etc. Ruger really offers their best options in 357 Magnum.
There's something else about Redhawks and SRH. It's sort of trivia at this point. There was a time when Ruger was using a chlorinated solvent or lubricant or cutting compound or something on the barrel threads of Redhawks. Apparently it embrittled the barrel, and the barrels would break off. While they were yet unaware of the cause, they designed the SRH frame with the massive lug for the barrel extending all the way to the front of the ejection rod. Then they identified the cause of the barrels breaking and stopped using that fluid. The SRH was already finished, so they went forward with it and the huge frame lug. Of course, another difference is the stem grip frame instead of the surround grip frame.