Add Jadis to the list too; their phono stages on the JP80 and JP200 are among the best.
I think you can explain the trend to separate phono stages cynically (hey, we already have the chasis from our line stage, now we can make more money selling a separate phono stage using that chasis) or on the perfectionist bent of the best designers, who recognize that in a no-holds barred design keeping the sensitive amplification of very low-level signals away from the line stage's power supplies and giving this stage its own power supplies is preferable to housing that stage in one chasis. I think the latter is more the case with the majority of the high end designers; I think that they believe that the vinyl lovers would likely prefer a higher level of performance from their equipment. Add to that the trend to make separate line stages that started with the rise of CD player sales, which would make designing a good phono stage to fit into that box a bit more difficult than designing one from scratch in its own box. Anyway, I think that may be an explanation, but as the above posts have pointed out, there are still plenty of excellent full-function preamplifiers out there.