If you are inquiring about the NX Class Enterprise, then that was the maximum rated speed the engines were designed for in the 22nd Century. Humans had not discovered how to go faster then that yet.
If you are asking about the Enterprise-D beginning in 2370, than it was discovered that warp drive up to that point causes enviromental damage. A Warp 5 speed limit was imposed on all Federation ships after that as shown in the episodes Force of Nature & The Pegasus. Click on the hyperlinks for more information. TrekFan1984 Ends (talk) 23:25, April 18, 2017 (UTC)