There are a few possible reasons for this:
1) Jewish composers may think no one will realize if they use tunes from non-Jewish songs.
2) Jewish composers may be influenced by some non-Jewish composers and incorporate some of their favorite tunes into their songs.
3) Some Jewish composers may not actually be talented enough to write their own material.
4) Some Jewish composers may be mostly after the money and use non-Jewish catchy tunes because they know people will like it, and therefore will buy their music.
5) Some Jewish composers are trying to reach out to people used to listening to non-Jewish music by using tunes they may be familiar with, to get them to listen to Jewish music.
There are probably more possibilities that I haven’t listed.