I couldn't find where it said that?
In the Iberian Peninsula, 128K models were all over the place.
Not really in UK or other European countries (besides the Amstrad almost-knockoff).
It is strange that on UK, Speccy birthplace they were apparently rare, given all the Crash and Your Sinclair magazines I owned at the time, and related content.
Amstrad bought Sinclair in spring 1986 and they ran down the remaining 128K+ stock, meanwhile the grey +2 was already ready to manufacture - they'd already designed the case and reversed engineered the 128K+ to get a head start. It was in the shops by that autumn.