Why does RNA adopt an A-form helix?
The beta-form of DNA and the alpha-form of DNA are based on the pucker on the sugar ribose. As DNA doesn't have a 2'-OH, it can obtain both conformations.



RNA does not have this luxury. The steric clash of the 2-OH with the 3'-OH makes the B-form to be very unfavorable thus constraining the RNA to adopt the A-form. Incidentally this steric limitation is also why RNA structures are more stable (see https://biology.stackexchange.com/a/769/389)