Question

why the cells of sclerenchyma tracheids become dead

Answer 1

It has to give its life to a greater cause: the support and protection of the plant body. Sclerenchyma cells get their name from "scleros," which is Greek for hard. These cells are hard because they have lignin in their walls (unlike Mr. Collenchyma up there) and are usually dead when functional.

Answer 2

Xylem trachieds, xylem vessels and xylem fibers are dead components of xylem tissues because they are highly lignified and scalarified. They, however, play very active role in conduction of water and dissolved salts and nutrients besides providing mechanical strength to the stem. This is the correct answer.