Imposed Speed Limit
Well, it was established that certain areas of space are negatively affected by warp drives, and so a speed limit of warp 5 was imposed except in extreme circumstances.
What happened to warp drives destroying the universe?
Just like a car, there is a cost to running at higher warp. The warp drive doesn't create energy out of nothing, it requires fuel ( Dilithium or anti-matter). You could cruise around at Warp 9 everywhere you wanted, but fuel economy would suffer.
Wear and Tear
Running the warp drive at maximum is like redlining the engine in your car. Doing that for sustained amount of time results in an engine failure.
Forced to extend her shields around the vessel to protect it from asteroid collisions, the Enterprise burned out three of four lithium crystals, forcing the crew to supplement with battery power. The fourth crystal subsequently failed, overstressed from handling all of the ship's power. ref memory-alpha
What is better, to arrive in the middle of the night at a destination, or when the captain will be awake and on the bridge? How critical is it to get there immediately (usually, not very). When there isn't a hurry, why rush to get there in 10 hours when you can get there in 2 days without any ill effects?