When I was in management they hated temporary income bumps to employees because when the income bump went away the employees tended to look for a different job at the income level they became accustomed to/dependant on. It's not they don't want to give you more income, it's they know they will be hosed when/if they cut your income later.

Edit: This was also an issue with year end bonuses. We did well a few years and bonuses were given and it made things difficult when the good years ended. I doubt they gave bonuses after that, good or bad year.