The link to the article below gives an analysis of some of the research carried out on this subject.
Only a relatively small number actually had Pernicious Anaemia. It suggests that if levels are very low especially if there are symptoms intra muscular administration replaces the levels more quickly and reliably to prevent further symptoms.
However, it suggests that 1% of an oral dose is absorbed by passive diffusion and for maintenance 1000-2000mcgs should therefore provide an adequate daily replacement. It suggests that if oral treatment is to be used blood B12 should be monitored closely to start with and then annually if and when levels are stabilised.
Since part of the rationale for using the less predictable oral treatment is time and cost saving for the doctor it is clear that the additional monitoring would totally invalidate that argument.
ncbi.nlm.nih.gov/pmc/articl...
These links also give useful information.