Why does it take so long for hot water to get from the water heater to the bathroom?

It takes as long as it takes depending on how far they are apart.Undoubtedly it is a long run. You have tow options. Installed a small dedicated instant on heater near the bathroom. You can also install a small pump that circulates the hot water constantly from the water heater to the bathroom. Either of these methods will give you hot water instantly. Contact a plumber for details.