You can't assign a letter to the partition, because there's no partition table.
the tool you used to create the WBFS partition deleted the partition table (which can define up to 4 different partitions), and your drive is one big partition. Windows expect a table to list the partitions and assign them letters (even if unformated or unknown format).
Some wbfs tools could format a partition from the 4 possible partition and keep the table, other tools was just overwriting the partition table.
I don't know if wbfs2fat works with no MBR (MBR = partition table).
Maybe wwt (Wiimm's WBFS tool) has some option to move the WBFS allocation table and generate a MBR?
there's not a lot of WBFS application to manage drive's partitions
wwt and wbfs2fat are probably the only ones.
I'll call
@Wiimm here, maybe he knows how to create a partition table on drives with WBFS partition starting at sector 0.
edit :
I don't know enough about WBFS format, but I think the first sector contains the WBFS header (12 bytes) and a disc table up to the end of the first sector.
1 byte = 1 game, 512 bytes/sector = 500 max games, 4096 bytes/sector = 4084 max games.
Then probably a list of allocated blocs for each disc entry, then the game's data.
So, it would require moving few sectors to create a partition table
sector 0 (WBFS header->MBR)
sector 1 (disc info start (lba aligned?)-> WBFS Header)
sector disc_data1 (moved first disc info to the end of the array)
if disc table 1 is used (disc 1 bloc 1->unused sector, set the new LBA in disc info)
that's the shortest method, else it would require deleting (or moving + fixing LBA of the first game) and rewrite the full header sector -> sector+1 using dd.