Scientists have certain concepts about naming their inventions and discoveries.Some use the alphabets in the same order. Some use certain other relevant generic names.
When a scientist has some doubts about some more findings to happen, he makes room for naming those findings also and names his own invention or finding accordingly.

Here Neil Bohr has used that convention. He has reasonable doubt that there may be some orbits within the orbits he discovered and also orbits beyond orbits he found. If any orbits are below his findings of orbits, then his present naming can accommodate them by taking previous alphabets.If some orbits are found after his outermost orbit, then they can be named by the subsequent letters. Thus there will be no clash or overlapping of names.
It was just a convention to avoid complications and convenient to all who are in the same research.