Why are the other bases not used for RNA capping and tailing?
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.