I think the reason is limited space in the overview. PSoC 3 is listed under "All PSoC Devices". Maybe series 4 and 5 are more advertised?
Also there are no obsolescense notices to be found generally. But I found, that Cypress cancelled the automotive PSoC 3 AXE and AXA? Last year as far as I remember they were found in the product road map as planned. What is the reason? They were interesting for many industrial applications (temperature range).
My 2 cents to PSoC3: The 851 was designed in 1980 which is quite a few years age when talking about computer tecnology. This you will see in depth when looking at the core: a stack of 256 bytes depth is making some standard programming techniques to a nightmare for the compiler builders. Compared to the risk-architecture of a moderately modern ARM core (M0 or M3) it looks like "a giant step for mankind". So for new designs my choice will always be on the PSoC4 or PSoC5 side. Especially because Cypress has made it quite easy to move the C-sources from PSoC3 to PSoC5.