I am wondering why the individual- and household-level CPSID variables are only available for the regular monthly surveys (vs. the ASEC)?
CPSID is not currently available for ASEC samples, because linking the Basic samples to the ASEC supplement responses is complicated by the fact that the ASEC includes an oversample of households from adjacent months. IPUMS-CPS is currently working on a variable called MARBASECID to facilitate linking the March Basic samples to their corresponding ASEC sample. See this answer for more information about MARBASECID.
For the years 2005-2014, it is still possible to link households across ASEC samples using HRHHID and HRHHID2. Prior to 2005, you should use HRHHID and HUHHNUM. To identify persons, you should also include LINENO, AGE, and SEX along with the household identifiers. Note that AGE should increase by one year.
Keep in mind that your match rate will be under fifty-percent for any given two-year period. First, only half of the sample is eligible to appear in both years of any two consecutive years. Second, households and persons can drop out of the survey (e.g. relocate, refuse to participate, etc.).
Hope this helps.