Firstly, the 144,000 are said to hail from the 12 tribes of Israel, and they have a definite number; conversely, the "great multitude" comes from all nations, tribes, peoples and tongues and they cannot be numbered. Granted, the kings and priests of Revelation 5:9-10 are said to come from diverse backgrounds too. But it seems odd for John to describe the 144,000 as Israelites in Revelation 7:1-8, then do a literary flip, as it were, and talk about the 144,000 as an indefinite and uncountable group from all tribes, etc.
While there are certainly differing scholarly views of Revelation 7:9, Robert L. Thomas thinks that META TAUTA EIDON "indicates a vision that is distinct from the preceding one" of Revelation 7:1-8. Furthermore, he reasons that the 144,000 and the great multitude cannot be the same group since "The earlier one was numbered, but this one is innumerable. One is exclusively Jews, the other is not. One is facing a period of wrath, the other has been delivered from it (Beckwith; Scott). This multitude includes far more than the 144,000" (Thomas, Revelation 1-7, page 484).
It has been suggested that the great multitude could be a heavenly group. Regardless of that debate, I just find it hard to believe that the multitude is numerically identical with the 144,000. Numerical identity is a concept that tries to explain what it means for two objects (A and B) to be the same. For instance, if the apostle named Paul who lived during the first century is numerically identical with the apostle of the same name resurrected to heaven by God, then the two "Pauls" are the same person. The above mentioned principle would similarly apply to the 144,000 and the great multitude, if they are numerically identical. But it's highly doubtful that they constitute the same (numerically identical) group.