Low volume sales. Almost all DAP manufacturers have to use old Socs.
There is no way any of SoC manufacturers will sell latest chipsets on such low volume sales. Cellphone manufacturers that make a lot of sales, Samsung, Sony, LG, Apple all get the latest hardware because there is demand.
You have to understand DAPs are such a niche product, 90% customer base is of audiophiles. That is such low amount of sales, and Chip Manufacturers are not going to waste their resources selling latest tech to such a niche product group.
Another thing is chipset support. For example Leadcore/Ingenic solutions are semi-conductor support companies that provide companies like Cayin coding support for providing firmware updates.
As
@Andykong mentioned, latest SoC chip is useless in a DAP if you cannot provide coding support to Cayin Developers/Engineers on how to provide firmware updates. A lot of coding required a Semi-conductor company to provide support for these mobile chipsets.
Hence niche products like these use true and trusted SoCs that get reliable support for firmware updates. Hence why you see Rockchip RK3188 in a lot of daps, there is support for those chipsets by these companies.
RK3188 supports Android 5.1 as that is only support given.