Well Mace, I think Ironically the main reason for the change over from Suitmation to CGI is the Same reason that Godzilla and Gamera were made with Suitmation vs stop motion.
If we Track back to the Production Of Gojira the film that would become the basis of Godzilla and lay the ground work for all the great Japanese Kaiju Movies and Television even to today's Ultraman and Super Senti. The Original Aim was to make Gojira using the same technique used the very very first Kaiju film. King Kong 1933 RKA Radio pictures USA. It was a bench mark, in the Giant monster Franchise not the first film to use the concept of a giant monster as far back as 1925 there were Giant monster films but it was the first in which the monster transcended from raw beast of destruction to real character. It was also widely successful it was a movie re-released 5 times across 3 decades and even into the 1950's it was a success.
So when the Great Ishiro Honda came in to make Godzilla it was not a huge leap for him to think Stop motion. The Original Kong was Stop motion and the effects were grand for the Time.
however troubles soon started the best FX man in Japan was the great Eiji Tsuburaya who also wanted to do stop motion but the budget and time constraints meant that they could not do the effects shots as they needed to be done. So they had to find another way that was faster and less expensive. Again Kong swooped in to the Rescue.
Japan had known Kong was a hit in the 30's and Japanese film producers like any of there breathren world wide were always looking for a buck looked to get the King Kong Cash. In 1933 and 1938 two early suitmation films Wasei Kingu Kongu and Edo ni Arawareta Kingu Kongu were made and Released in Japan. Both Films were lost in the chaos of WW2 but technique used was the basis of Suitmation take a stuntman dress him as you monster and let him loose on scale miniatures.
Eiji Tsuburaya was already a master of miniatures he had done extensive work with them in movies before the war and in propaganda films during it.
He found ways to turn the fall back plan into a advantage with the right filming and actors.
It was Cheaper, Faster to film and effective. A sculpted suit weighing almost 200 pounds 6 foot tall and made of black Rubber was the answer. It combined with some puppetry and a exceptionally built studio miniature.
Suitmotion was launched and found it's way into Successive Japanese productions as well as a few foreign ones to. American TV and movies made use of the technique either totally or partially using actors on miniatures or suit-man in sets. Attack of the 50 foot woman or the battle or Kirk vs the Gorn in the Star Trek Episode Arena.
Even Today some degree os Suitmotion is used in Super Sentai. however for some time now Godzilla productions have blended CGI and Suitmotion. Even the terrible 1998 Roland Emmerich film used some Suit motion mostly for the man sized Zilla spawn. And Godzilla 2000 featured a number of CGI FX inter cut as did successive godzilla films and Gamera films of the Same era.
And in A way Suit motion was also used in Godzilla 2014, Motion Capture was used in modeling the movements of Godzilla in the film.
Because CGI is not as destructive IE if a building comes down wrong in the fist take you can rest with a key stroke and not days of work down the drain it's cheaper. the details are unlimited and the effects of the monster action can be blended more accurately It seems likely that from this point on true full Suitmotion films are gone. what will remain will be use of motion capture some puppetry and blended practical effects into CGI.