From the perspective of government, speed limits are the easiest way to go. Being over the limit is a strict liability offence, the "crime" is easily provable and easy to collect evidence for - and the most corrupting aspect - easy to profit off. However, being a strict liability offence, it makes no distinction based on the environmental circumstances of the road. I can think of a stretch of road that is as wide as a freeway, 2 lanes both ways, where the limit was lowered to 80kmh/50mph. Depending on the time of day, the road can be packed or almost empty. Naturally, at the times when its almost empty, and traffic naturally flows faster, is exactly when the gubbermint likes to put out its privately contracted speed vans.
The spectre of money really poisons everything. Its been an ongoing debate here in Australia. There is a very sordid web when it comes to lowering speed limits, the increasing use of private companies for speed enforcement, and the use of taxpayer money to propagandise the issue. For example, Monash University Accident Research Centre has ongoing controversy surrounding it. It produces studies to support lowering speed limits, whilst having financial ties to the private companies that operate speed cameras.