Previous answers have really nailed the most common reasons for this frequently encountered situation, so why don't we prattle a bit on what you can do about it.
IF GIVEN a situation of natural/typical distrust between the front and back offices (well described above), you'll have to actually do something to break out of the pattern. This is easier to do in SMB than enterprise situations.
A couple of things will generally get the attention of management and provide a basis for some trust to form: (1) show real interest, and (2) show real business sense.
This isn't as hard as it may seem, but it will require a change of mindset. (Suspend any beliefs that 'users' are 'stupid'.) Get some time with management and ask how the business works. How does the firm make money? How does it differ from its competitors? What are the top three problems management is trying to overcome? Note: don't talk IT here - and if they try to steer the conversation that way, put them back on track.
SHOWING BUSINESS SENSE
Assume that your budget (if you have one) is funded by YOU; be very, very careful about how each penny is spent. Before purchasing something, line out the Pros & Cons to the business of (a) acquiring the asset and (b) NOT acquiring the asset. (Hint: some call this the business case.) Ask management to review it before making a request for purchase.
Put effort into demonstrating the stuff you are retiring from inventory and/or how you are saving the firm money. Again, be very transparent and unemotional. Ask management to review your report and provide feedback.
In SMB the 'distance' between the decision-makers and IT doesn't have to be great. But, it is up to the IT staff to come out of the cave on a sustained and consistent basis.
It works and you'll be a better resource for it.