Why I cannot find dendritic cells in blood smear?

The dendritic cells are antigen presenting cells that process and present pathogens to the T Cells.

As such they are present in interfaces where foreign organisms are frequently present. Eg: Skin.

When these cells get activated i.e. come in contact with pathogens, they process them and move to lymph nodes to present the processed antigen.

Thus these immune cells do not circulate in blood once they reach the target tissue but reside in the tissues and found in lymph nodes on activation.

Further, from Kuby's Immunology:

Dendritic cells can be difficult to isolate because the conventional procedures for cell isolation tend to damage their long extensions. The development of isolation techniques that employ enzymes and gentler dispersion has facilitated isolation of these cells for study in vitro. - Kuby Immunology 5Ed. P.42

Since blood is the most accessible tissue for clinical studies, we set out to extend the findings that were reported in mouse blood to humans. However, when we tried to induce DC growth by adding GM-CSF to human blood, we identified actively proliferating DC aggregates only infrequently - Proliferating Dendritic Cell Progenitors in Human Blood By Nikolaus Romani, Stefan Gruner, et.al.