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As a kid, I always had problems with the keyboard. Membrane was hard to come by and expensive. I did not have Kempston/Protec. So I had to play on the keyboard.
I was using QAOPMN combination, and shortly it would wear off. Then I had to try to fix it manually, by putting wire and what not. Usually did not work.
In any case, I always wondered why Sir Clive chose the membrane. I suspect it could be to cut down the production cost, but I am not sure.
On the one hand it was clearly down to cost - being very much cheaper than a conventional keyboard.
However, remember that the Spectrum was the replacement for the ZX81 and ZX80. These had absolutely no tactile feedback in their capacitive keyboards and so the Spectrum could be marketed as a great leap forward in quality and user experience. Although that claim was open to debate it did create sales. A small increase in price gave a large increase in perceived quality and usability.
Also, the membrane keyboard enabled the Spectrum to differentiate itself from competitors that were beginning to appear in the ultra-cheap home computer market from the likes of Oric and Tangerine which had the ZX81-style of capacitive keyboard.
As simple as that: because it is the cheapest solution. Mechanical keys, separate circuit boards to hold the switches, etc, add costs to the final product. The ZX Spectrum case is slighly bigger than the circuit board itself, and thick enough to accomodate chips with sockets (there is no room even for a heatsinked socketed ULA).
In addition to manufacturing costs, a "proper" keyboard would surely have caused the case to be bigger, so the polys and cardboard box would have been bigger too, adding more weight (both real weight and volumetric weight, which comes into importance when you have to transport many computers) so the smaller the computer, the smaller the costs associated with storing and delivering them.
It was all about cutting down costs, anyway.
I had a ZX81 before I had my Spectrum. I am sure I am not alone in being grateful for the huge improvement, at minimal increased cost of the Speccy compared to the 81.
The 81 membrane was very fragile.
So a membrane with rubber keys as movement, and feedback to the user, also added protection to the membrane.
Clive produced hardware that traded off price and reliability, to bring consumer electronics to the masses.
Still, the most common failure of an old spectrum is the membrane. I had to replace mine. They are available on SellMyRetro and elsewhere online, with instructions. Or Mark Fixes Stuff has excellent YouTube videos to show you how.
