Why does the addition of a 5' cap and that of a 3' tail involve guanine and adenine respectively? Why aren't any of the other two bases added to an mRNA to protect its ends or act as a signal to transport it to the cytoplasm?
I don't think definite answers exist but I can think of theoretical reasons:
First, Poly A tail can efficiently be synthesized because ATP is the most readily available nTP, being the main energy source.
More to the mechanism, but this is no explanation to the evolution: The termination sequence 'TTTATT' on the DNA template and 'AAUAAA'on the RNA is the start of poly-adenylation. But this falls in the 'chicken-egg problem' category.
As a bioinformatician, I happen to know, that 3'UTR is highest in A-content of the 3 genomic contexts and the 5'UTR is highest in G-content of the 3 contexts, so possibly these post-transcriptional modifications further strengthen the compositional differences of the contexts.
For the poly-A tail it might also be relevant to mention that G or C sequences have higher propensities towards structure due to amount of hydrogen-bonds.
I hope there were some helpful pieces of information.