why do some manga's just stop completely, it's not like a television where you can go bankrupt if it fails.?

It's either the mangaka (author) dies, manga sold poorly, plagiarism/copyright issues, manga's popularity is low and/or long hiatus by the authors.

Some example of these would be manga in Shounen Jump

If the popularity of the manga is low, the manga will be canceled or stopped. Same with the sales also.

Itazura na Kiss series was even unfinished due to the death of the mangaka from an acccident. Kaze no Stigma and Zero no Tsukaima (even though these are light novels) are also victims of this factor.

Plagiarism or copyright issues example would be Aflame Inferno (stopped indefinitely due to copyright issues) and Incarnate (this is a comics though which was hinted to plagiarize some manga works like Bleach)

Some titles that have been on long hiatus could be next victim of cancellation. I can only give Advance of Zeta: Toki ni Aragaishi Mono as an example but a factor of this was also the deteriorating health of the mangaka. Some 'on hiatus' manga (even though it's been really long) won't even be canceled like HunterxHunter and D.Gray-man. Probably due to its immense popularity.

If you want to see the process of these. Try watching the anime Bakuman (you can also read the manga).

It shows how popularity and sales can affect the manga.