I think there is rather simple explanation. Most people in Poland to watch movies on PC use very good polish players/subtitle editors like:
Subedit Player -
http://subedit.com.pl/
BestPlayer (older) -
http://www.bestplayer.com.pl/
This soft is very easy to use, works with subs formats: MicroDVD, TMP, MPL2 (and some more) and can autodetect format, so this is enough to put subtitles in *.txt file
Most movies are in DivX/XVid, and nobody hassle with converting it back to DVD. Even many cheaper standalone DVD players sold in Poland reads DivX/XVid and subs from *.txt.
I think that formats like MicroDVD, MPL2, TMPlayer are much easier to use from programmer perspective, and for translator/watcher too. They are easy convertable too.
EDIT
One more thing, meybe the most important: polish character encoding. Most popular players lacks support for it, so we had to write our own, thats made formats they use most populat, and now this is a habbit
.