It depends on whether you're talking about horizontal or vertical centering i.e. textbox items can grow only in vertical direction and thus they cannot be centered vertically, as they would grow dynamically based on the provided data.
The Crosstab item can grow both ways - horizontally and vertically. You can center it in the report designer, but if you have dynamic row groups and dynamic column groups, it would grow in runtime in both directions and thus it would not be centered.
All the best,
the Telerik team