Most of the studies are measuring mainly serum CORT levels and rarely measuring serum ACTH levels. which is better indicator of depression and how exogenously administered Corticosterone causes depression as it is having low half life.
Generally, steroid hormone measure comes with fewer potential complications than protein hormone assay. The highly conserved nature of steroids makes finding an antibody relatively straight-forward whereas this is not always the case (depending on what animal one is working on) with peptide/proteins. For example, I (and several others) had a most difficult time finding an AB for prolactin in dark-eyed juncos (Junco hyelmalis) ... amino acid substitution(s) may result in a lack of cross-reactivity. There are likely other factors that play into your observed bias including turn-over as you mention with the relatively short half-life of ACTH being a potential complicating factor that would need to be controlled.
I think part of the bias might stem from the simplicity of measuring corticosterone, even in "crudely" prepared serum samples, with commonly available assays (ELISA, RIA). Corticosterone is a fairly stable molecule, whereas ACTH is subject to proteolysis and generally considered to be less stable. After a lot of reading and many discussions with colleagues, I decided to try and measure both corticosterone and ACTH in mouse serum after stress. I used aliquots of serum from a given mouse for within-subject measurements of the two hormones, and it worked rather nicely (see Stroth et al., J Neuroendocrinol. 2011 Oct;23(10):944-55).
Note also the following from Reisch et al., Clin Chem. 2007 Feb;53(2):358-9: "In our study, for up to 24 h the decline in the measured ACTH concentration was ≤10% even in whole blood stored at room temperature." This suggests that ACTH is not as labile as commonly assumed, so that one should be able to derive meaningful measurements from a well-controlled experiment.
In a given study, corticosterone might also simply be the better indicator of what the experimenters are looking for. ACTH and corticosterone don't always go hand in hand, i.e. can be dissociated (see Bornstein et al., Trends Endocrinol Metab. 2008 Jul;19(5):175-80).
Finally, measuring corticosterone rather than ACTH might be a matter of convention in a given field. Sometimes people measure what people have always measured, just because "that's how it's done."
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