Sometimes to prevent ease of travel for homeless or other loitering activities, sometimes because everyone wants to use tax money for their pet projects and it’s hard to excite people about sidewalks. These are just my guesses
It’s not a priority? In my city, it’s up to private developers to put them in, so we have a hodge podge of alternating sidewalks with surprise endings.
Neglecting sidewalks is nearly the same as not building them. The sidewalks in my area can be terrible even if the road has recently been resurfaced. There are many locations where wheelchairs, strollers, and walkers would be difficult to use to due to lifted, cracked and chipped sidewalks. Some sidewalks have over grown plants to the point that they are nearly unpassable. In some areas this requires stepping off the side walk and into the oncoming lane and around a blind corner. Even consistent paint at crosswalks is not provided with many intersections having no crosswalk lines or only crosswalk lines in 1 direction.