It's just the order the approaches were created. From the TERPs manual:

"CIRCLING PROCEDURE IDENTIFICATION. When an approach procedure does not meet criteria for straight-in landing minimums authorization, it shall be identified by the type of navigational aid (NAVAID) which provides final approach guidance, and an alphabetical suffix starting with the beginning of the alphabet. The first procedure formulated shall bear the suffix “A” even though there may be no intention to formulate additional procedures. If additional procedures are formulated, they shall be identified alphabetically in sequence, e.g., VOR-A, VOR/DME-B, NDB-C, NDB-D, LDA-E, etc."

Also if another airport in the same city had a circling approach of the same type they would not have the same letter. So one airport in town could have a VOR-A, VOR-B and VOR-D while an airport cross town would have the VOR-C.