With the latest administration, emissions seems to be much less of a factor given the crazy reverse course every US centric manufacture is taking right now.
It's about the development cost - it will be much cheaper to improve upon the LT4 and certify it then it would be to take a brand new engine and FI it for the first time, all to be sold on what is a very low volume production car. Given that the Corvette has its own stable of FI engines, GM can no longer "double dip" like they were doing in the 2010's. If you recall, the LT4 was first introduced on the Corvette Z06, then the Camaro ZL1, then the CTS-V after that. The Corvette is a (comparably) much larger volume seller that they just don't have anymore with the flat plane crank + TT engines they now use.