Sometimes there isn't enough physical space for the ILS components.
Also, perhaps the winds are out of the other direction when there are low ceilings.
More the former than the latter, but it's a consideration.
Sometimes the case, yes.
Could literally be a million reasons from noise abatement procedures, terrain for that ILS, setting up approach lights, federal funding for ILS approaches, etc.
Exactly. Most of the time, it has to do with meeting the FAA's requirements and there are many. Often, it's an issue of runway length and approach clearance (glide-path gradients). Sometimes, as above, it's related to obstacle clearance - there are several zones for the precision system that must have certain clearances. Outward from the OFZ, there's a conical surface defined by TERPS. It surrounds the runway, ascending on all sides, but the most important sides are the ends, obviously.
This is what it looks like when you plan it all out. Mind you, this is only for a LOC, but it's similar (just a bit more lax).
http://www.leesburgva.gov/Services/airport/LayoutPlan/2007_Layout6.pdf
The airport depicted is JYO, which I didn't work on, but I worked on a few others when I interned with Cambell & Paris. You can actually see the orange lines on the diagram saying "Future 50:1 Approach Surf," which hints at a future ILS. We dropped Leesburg's contract at the end of the term, so I'm not sure whether or not they still plan on it.
If you have any airport planning-related questions, shoot one at me. If I don't know it, I can always forward it on to the guys I worked with.