We cannot identify the type of tree in the photos. Once a tree is dead it is difficult to determine why. Your tree looks like it has mechanical damage to the trunk, may be planted too deep, and there is excessive mulch around the base.
In general trees decline from any type of stress such as drought, poor drainage, too much moisture, mechanical injury to the trunk, planting too deeply, excessive mulch, etc.
Take a look at our publication on these types of problems https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs...
When the trees suffer from the above they become weakened and more susceptible to possible disease and insect issues. You need to replace the tree that matches your site conditions and consider mature height and width. Take a look at our website for our video, the planting process, and after care https://extension.umd.edu/hgic/topics/planting-process