Possibly because there is blockage in the pipe somewhere. You haven't said if you are getting water at all in the hot tap. Is there just no hot water, or no water at all. you need to check for leaks around the H/W tank,and also ifthe whole house water pressure is too low.
Check aerator screens at the end of faucets and any water saver screen/restrictors in the shower heads first, toilet valves can also become clogged and slow over time.
ANS 2 If you noticed water pressure dropped upstairs, it possibly also dropped downstairs and you haven't noticed. -If you are on city water, check the PRV. (old PRV's will reduce pressure) If on a well, check pressure at the tank. (possibly a waterlogged tank) Just looking at flow isn't always an indication of pressure drop, - if possible measure it on a gauge. There are gauges that will screw on temporarily where any aerator fits.( I have one - less than $ 10 at Plumbing supply store. )
MY upstairs water is working fine, why do you ask?
Is the water not hot? Or no water coming out the hot tap? not comming out at all
Check the upstairs taps for blockage in aerator or internally.
Because the water heater is downstairs, and the pipes are poorly-insulated.
it sounds like there are crossed lines. if there is no water upstairs there has to be a valve that is off. even if there is low pressure it would eventually fill the line and you would get some water.
Bad shower valve.
Gravity works against hot water heaters. Generally they are on the main level of the house. The hot water has to flow upward to reach upstairs bath fixtures. Some owners invest in a second water heater which is placed on the second level. The main danger with this is a leak. Some consider this danger too great. Others see the cost as prohibitive. However, some home advice experts recommend a second water heater. By adding an upstairs water heater, home owners decrease water and power usuage. They don't use as much water because it is warm on the spot.
Temperature balance shower body needs to be adjusted
Because heat rises and obviously upstairs is higher than downstairs so the heat will rise to upstairs.
Heat rises.
If the plumbing in your house is old, it is probably rusty water pipes or rust from within the Hot Water heater. Des Perado
Replace the tub faucet.
turn valves off at up stairs heaters