They also break more easily (See the "Oreo" problem on first Palm Prē) which is costly nightmare in logistics (returns, warranties, etc.)

And beside us geeks (who need to type code, shell, or use complex technical vocabularies, or multiple language that defy usual input tools) most of the other "normal" users can do with simplified input tools (autocorrect or swype) with the occasional small bluetooth keyboard.
So there no real point for a big company to invest the costly extra resources in producing slider keyboards, it won't attract that many additional users. Better invest in cameras, *that* one attract more users.

Also, slider keyboard add a little bit thickness, and given the current trends, having a smartphone thin enough to slice cheese with it is a fundamental need for everyone...

Thus, big thanks to chen to thinking about catering to the few with different needs like us.