Why is there only IPUMS CPS earnings data for March sample?

I am trying to use the IPUMS ind1990 continuous industry classification with the standard outgoing rotiation survey sample for studying wages. However, the wage variables available in IPUMS CPS show missing data for all months outside of the March supplement. So, the wages sample I end up with is only drawn from March, not the whole ORG, as in the NBER files. What am I missing? Thanks so much for your help.

IPUMS-CPS does not currently make all of the Outgoing Rotation Group questions available as part of the non-March Samples. However, IPUMS-CPS is working on expanding the availability of these variables as a part of the increase in basic monthly samples and variables. In the meantime, it is possible to merge data from the non-March NBER basic monthly data files onto the corresponding IPUMS-CPS samples using a sequential merge. See this question for more detail.

I hope this helps.