Did we? I dare to say that your historical account is inaccurate. We stuck to our traditional layout for a while. What does it matter if the keyboard wasn't customized in physical sense anyway? You should be looking at the screen, not at the keys. If I recall well, Polish typewriter layout (PN-87) was still available on Windows 95/98. The now prevalent >Alt + something< one was called "programmer's layout", and the name itself indicates that it wasn't originally thought of as everyman's layout.
Domestic software certainly used the typewriter layout - for instance http://pl.wikipedia.org/wiki/TAG_%28edytor_tekstu%29, once a very popular word processor for PC, last version released in 1996.
The typewriter layout finally got extinct, but what spelled its ultimate demise was the internet, I believe.
Early netiquette actually forbid using Polish diacritics, because of encoding issues - in that era you could never be sure whether the other person would read "gwóźdź" or "gw░¶d¶", so it was considered good practice to stick to Latin characters only.
Meaning that users didn't get to feel the pain of having to press Alt + whatever all the time, and so they got hooked on default QWERTY.
As the last of Mohicans, I use the typewriter layout to this very day (for typing in Polish; I alternate), only I had to recreate it myself (using Microsoft Keyboard Layout Editor)*
It allows for much faster typing in compare to these inconvenient right-Alt shortcuts. Swapping Y with Z by itself is a win for a Polish speaker (writer), given that Z is much more common in Polish. While its frequency is at mere 0.07% in English, it reaches 4.9% in Polish, placing it in the top 10. Thus putting it under the weakest finger of all - your left pinky - isn't very considerate.
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* Funnily enough - I'm not flame baiting - when I decided to try Linux, hailed for its customizability, I found out that recreating my favorite layout wasn't as easy. I got lots of advice on various forums, but noone had a simple receipt for me. Admittedly this was few years back.