Enslaved people had already been emancipated, they just didn’t know it.

Abraham Lincoln issued the Emancipation Proclamation on January 1, 1863. The Emancipation Proclamation freed slaves in states and then in rebellion against the United States. The Proclamation applied to more than 3.5 million of the 4 million enslaved people in the country. Around 25,000 to 75,000 were immediately emancipated in those regions of the Confederacy where the US Army was already in place. It could not be enforced in the areas still in rebellion, but, as the Union army took control of Confederate regions, the Proclamation provided the legal framework for the liberation of more than three and a half million enslaved people in those regions by the end of the war. Union troops operating in said states gave teeth to the Proclamation. This, however, did not apply to the border states.

The Juneteenth announcement came more than two-and-a-half years after Abraham Lincoln issued the Emancipation Proclamation. So technically, from the Union's perspective, the 250,000 enslaved people in Texas were already free, but none of them were aware of it. During that period of time, they still lived how they did before, which was as enslaved people. Even though the people in the upper class knew, no one was in a rush to inform them about the fact that they have already been emancipated.