The virus evolved in bats, which have a much faster metabolism (to support the metabolic requirements of flight) and a constitutive (always on) immune response.

The virus evolved in bats, which have a much faster metabolism (to support the metabolic requirements of flight) and a constitutive (always on) immune response. This means in bats, the virus is always "on the run" from the host immune system and the host may never actually completely extinguish the infection, allowing it to continue at a low level while reducing inflammation. The trade-offs and selective pressures are different in the bat host compared to human. Packaging more copies of the mRNA per particle implies slowing down particle production so more mRNA can be made, but in bat that may slow down the generation time so much that the virus gets extinguished by the always-on host immunity. That's just one plausible mechanism of many, including simply that the protein structure was just not as "optimized" as you expect prior to the spillover event.