I would re-phrase the question as "Why WOULDN"T the appraiser 'use an active or sales comp for rental information'?"
Exactly. Rental comps are used to determine exactly one thing - rent. For this reason the most recent and accurate info is the most important, and a property that sold 6+ months ago would tend to have less up to date information then a property that was just listed last week.
Sold comps are used to determine GRM.
In a small income property, 2-4 family, the income approach is often the most reliable approach to value.
In my CR class the instructor point blank informed us that the income approach is rarely (if ever) the most reliable approach for SFR but for multi-unit apartment complexes (5+ units) it is almost always the most reliable (rarely are others used for the largest apartment complexes), and that for 2-4 units the income approach slowly becomes more reliable and dominant as the number of units goes up. The reason is that (at the time) most duplexes were purchased by owner-occupants but few if any 4-units were, thus the typical buyer motivation and intent shifted.
Since then at least one local market (Kenosha) has shown a major shift where even duplexes are (apparently) being purchased primarily by investors and thus income approach has become dominant for many duplexes as well.
I thought I would add that as the motivation of the typical buyer can shift (as can who the typical buyer is and in what ratio) and thus can affect which approach is likely the most reliable based on the current typical buyers for that property (in Kenosha there are different buyers (strata) from gutted REOs, barely uninhabitable REOs, barely uninhabitable non-REOs, barely habitable REO, REOs in average condition, barely habitable non-REOs, REOs in good condition, avg condition non-REOs, and good condition non-REOs (usually in the superior areas but not always) in the duplex and separate 3-4 unit sub-markets. OK, Kenosha is a strange market

with all those different strata and buyers from rehabbers to landlords to (very few) owner-occupiers all still active in the market (with investors being the most active, esp. both rehabbers & landlords).
Good luck &
