It's likely to be due to a combination of factors, some of them mentioned by the member above. The emergence of online sex work as an alternative (e.g. OnlyFans) is often identified as one, although some observers, myself included, doubt that it is a significant factor, because earnings are meager for the vast majority of "creators".
I am going to put forth a new hypothesis to explain in part the low number of new escorts working for agencies in recent years: the clients. What if the quality of clients has declined so much that the word is spreading that working in agencies is not "worth it" anymore? (see note 1). To illustrate my point, I wil use the example of public transit, because this is a place where all social classes co-mingle, and because all social classes (except the very poor) patronize escort agencies. What do we observe in public transit? An abundance of bad manners. Feet on the seats, loud conversations of speakerphone, bad body odour, people not giving up their seat for the elderly or disabled, etc. As a long time (20 years) user of public transit and social observer, I can assure you all of this has become much worse over the past 10 years or so. I have also heard first hand complaints from escorts about their clientele, bad hygiene being the main, but not the only one. This may lead women who wish to engage in lucrative sex work to enter the world of SB/SD arrangements, where they exerce a much greater control on who they are dealing with (see note 2).
Notes:
1) One of the main ways women join an agency is through a friend who has worked at the agency.
2) On the main platforms for independent escorts (Tryst and IndyCompanion) one sees almost only experienced escorts.
