Hi, Rich.
My current research showed me, that it's because of value in field RSDS-VIRTCUBE.
I've tried to activate mentioned extractor 2LIS_11_VAITM in R3 and replicated it to my BW, but it didnt appear in available DS'es for template. Then I've looked at all active datasources in table RSDS (where OBJVERS = 'A' and OBJSTAT = 'ACT'), and have seen one additional record for this extractor (in template selection there was 68, in table - 69). I've checked "interesting" fields from left to right, and the first was VIRTCUBE: all visible extractors have a value '1' in this field, and 2LIS have '' (empty) value. Then I've changed it directly in table to value '1' for 2LIS and it became available in selection window.
The conclusion is: you can select only those DS'es, that have a "Direct access" property of the DataSource set to "Allowed" or "Allowed with pre-aggregation" (value 'D' removes DS from suggestion list).
The next step for you is to investigate, why did SAP make things in such way
Regards,
Andrey.
P.S. My BW is SPS10 and R3 is 46c.