My Turn
There are some very appropriate mechanical reasons Percussion Colt revolvers were not converted in the same manner we think of. The first restriction is the basic frame size. ALL 36 and larger Colt percussion guns were built on the same base 1851 frame. There was no real room for expansion.
When conversions became possible (White patent expired), the dimensions of the cylinder were constrained by the original design (1851) dimensions. The exception was the rebate to the Water Table to allow the stepped cylinder of the 1860. The back of the 1860 cylinder was still the same dimension as the .36 cylinder.
Converting to .45 was never and option in the 19th century. Those guns had Iron Frames and Iron Cylinders. There was insufficient cylinder wall thickness to contain 45 Colt Cartridges and pressures. The only large bore option was the 44 Colt cartridge with heel base bullets. The smaller diameter of the 44 colt cartridge with it's minuscule rim allowed 6 cartridges to fit without interference of the cartridge rims and the recesses in the Star. It must be remembered, the 44 Special and 44 Russian are modern iterations of the 44 not seen in the halcyon days of yesteryear. The modern 44 Colt also has a much differs case. The rim of the modern 44s is not far removed from the rim diameter of the 45. A very difficult "fit" in guns of the 19th century.
Fast forward. Today dawns along with reinactors and CAS. CAS shooters clamoring for non historically accurate reproductions of 19th century guns. Up jumps UBERTI of Italy. A golden opportunity for profit appears. Uberti scaled up the size of their 19th century reproductions to just barely accept 45 Colt cartridges. In fact, it the early days, it was not uncommon for Uberti built Open Top reproductions is 45 to spit out the bottom piece of metal (super thin) of the cylinder locking bolt slots. As a Gunsmith, I was never in favor of 45 Colt in the reproductions of 19th century guns. Even in modern materials, the cylinder walls are just too thin. Anyway, the increase in size of the cylinders also allowed the modern rim size to load without interference all the way to 45 Colt.
I don't personally recall any Gunsmiths converting Colt pattern 19th century guns to 45 Colt. I knew Robert Millington and Robert wouldn't attempt it with a 6 shot cylinder. Remember, Iron Frames and Iron Cylinders do not mix well with modern smokeless powders. Even todays conversion smiths won't put 45 ACP in conversion cylinders because of the high pressure of the ACP round.
This has been the condensed version of "WHY" in this case.
We now return to our regularly scheduled programing.
Hide and Watch
PS: I forgot. 19th century .44s were not 44s. The bore diameter was actually that of a .45. Heal base bullets allowed that diameter bullet to be loaded in a 44 Colt cylinder (smaller than a 45). Stuffed into a 44 Colt case which was smaller in diameter than a 45 cartridge. Today's .44s are 44s. Well not actually. Today's 44s are actually .43s. Oh, and to muddy more, Todays .38s are really 35s so go figure. Cause .36s are actually almost true 38s.