Katakis was created first! The company that owned the rights to R-Type (Activision I think) thought that the elements in Katakis were a bit too similar to the arcade classic so took legal action to stop distribution.
Rainbow Arts then wangled it so that with some graphical amendments the game could be released (this is the version call Denaris) To allow them to do this the programmers had to produce the R-Type home conversion on the C64 in a very tight timeframe.
If I was in that position I too would have turned out a lame conversion if only to sell more copies of the superior rip-off.
Anyway a very good game, but there are quite a few shmups on the C64 that are better.

(taken from lemon64.com forum)