Where have all the transistor portables gone?
Gone to trash dumps everywhere.
When will they ever (re)turn?
When will they e-e-ver (re)turn?
Apologies to Mr. Dylan. 
So many things like this just gradually fade from the garage sales, flea markets and thrift shops... I can remember when most flea markets had at least one office "dictaphone"; haven't seen one in years. Remember metal lunchboxes with current TV shows on them? Don't see many of them any more. Plain, blue denim-covered 3-ring notebooks like most schoolkids used to carry? All plastic now. How about clock radios? Telephone answering machines using cassette tapes? Big desktop calculators (esp. the plug-in type, let alone the old mechanical ones!). Typewriters are getting rare. VHS taoe rewinders shaped like racing cars. Old hand-cranked pencil sharpeners that screwed to the wall. AM-only and pocket transistor radios, as you say. Those "executive look" telephones in a faux-woodgrain and black box that sat on a desktop. Portable black-and-white TVs in rounded shapes and orange colors. All these and many, many more things (that I can't remember, of course) are just gradually disappearing from the planet, and soon from collective memory... save for the "memories" of collectors who nostalgically cling to a few.