I think it’s because of past history with mRNA vaccines, low sample size of current Covid related tests, effectiveness that wakes over time which will lead to the need for multiple boosters, plus encoding + including the full spike protein could lead to serious adverse events that won’t be known for a longer amount of time.

“Limitations of our clinical study include the small sample size and its restriction to participants below 55 years of age. Another constraint is that we did not perform further T cell analysis (for example, deconvolution of epitope diversity, characterization of HLA restriction, T cell phenotyping and TCR repertoire analysis) before and after vaccination, because of the limited blood volumes that were available for biomarker analyses. Similarly, we did not assess the induction of tissue-resident memory CD8+ T cells. Further, as vaccine-induced immunity can wane over time, it is important to study the persistence of potentially protective immune responses. Samples to assess persistence are not yet available but are planned in the study protocol and will be reported elsewhere.”