It's Slartibartfast, not 'Startibartfast' and this is what Wikipedia says:
Douglas Adams writes in the notes accompanying the published volume of original radio scripts that he wanted Slartibartfast's name to sound very rude, but still actually be broadcastable. He therefore started with the name "Phartiphukborlz", and changed bits of it until it would be acceptable to the BBC. He came closer to achieving this goal in the following episode, with the double-act Lunkwill and Fook. He adds to this statement in Don't Panic: The Official Hitchhiker's Guide to the Galaxy Companion, an analysis by Neil Gaiman:
...One thing I don't think I explained in the script book was that I was also teasing the typist, Geoffrey [Perkins]'s secretary, because ... she'd be typing out this long and extraordinary name which would be quite an effort to type and right at the beginning he says 'My name is not important, and I'm not going to tell you what it is'. I was just being mean to Geoffrey's secretary.[4]
Even in its 'acceptable' form, the name sounds silly, and suggests flatulence (farting) to our childish minds.