You shouldn't get character philosophies confused with the author's philosophies. Detective Conan is written with a certain demographic in mind. So writing everything in black and white when it comes to morals is a necessity. I'm fairly certain Gosho not only recognizes, but would use the gray area if his target audience was not kids.
So, yes, in DC the two main detectives in DC are idealists, and have ideological philosophies. However Gosho has proven that suicide is not out of his realm of ability. He has done it twice. The first with the Yoko Stalker, and the second with Moonlight Sonata.
You also have to take into account that it is far easier to write mysteries for murder than suicide. Please refer to the TV Tropes page Always Murder for more information about that.
In the end, it isn't that Gosho doesn't want to write about suicides, because he has. There is likely more complicated reasons that you would have to ask Gosho himself. However, I would suspect that it has something to do with the morals he wants to convey through Detective Conan (killing is bad, even yourself), and it is easier and more interesting to write about murder than suicide.
Edit: replying to Shin-chan and OP
This is also true... This has to do with the perception of suicide in the culture. In Japan suicide was, and in some ways still is, seen as an honorable way to end a disgrace. I know there are more than a few ghost stories in Japan that have to do with suicides.
It all comes down to perception. In the west, suicide is frowned upon, and always has been. Even in our equivalent of feudal times (medieval) suicide by a knight was seen as cowardly. So naturally our literature reflected this and suicide was less prevalent than murder. Which is why most LITERATURE is mostly murder and robbery mysteries in the west. Where as in Japan deaths were quite often suicides, because of Bushido. So for them it would be more interesting for it to seem like a murder and turn out to be a suicide which was more believable.
How this affects Gosho is another story. I cannot personally confirm that the prevalence of suicide based mysteries in Japanese literature lead him to go mostly for murder, but we do know that Gosho is influenced by both types of mystery, Japanese and Western alike. If I were to guess his motives in how he proceeded, he used those two suicide cases to set up to refer back to meet his own goals for the moral standing of the manga. As I said in the relpy to Shuichi Akai, Gosho is very clear in that he wants to make killing of all types seem bad. Had he not had those two suicide cases, he could not have done so.
Hell, the only criminal to not meet justice is the one thief in his series. Had he wanted all crime to be called into his moral goals he would have had at least another thief (one which he did not care so much about as KK) and have them arrested by the intervention of Kogorou/Conan (setting up pretense for that would not be hard with a private detective).
As for OP, NCIS and CSI are also totally different sorts of mystery shows. A better comparison would be say Psych or Monk. They show more the science than the detective work, and they make their arrests based on that, where as Psych and Monk aim to show the abilities of the detective, and thus they never really make it to the lab, or it is less shown. And on that note... I will humbly request that you cease and desist watching CSI... They destroy science and the public perception of how Crime Scene Investigation works. NCIS sorta does too, but they are at least closer to how it really works.
PS: I think I'm done now