It's simple really:
-If you know how to tank a fight, then you know how to DPS/Heal it.
-If you know how to DPS/Heal a fight, you do NOT know how to tank it.

*Note: Healers can transition to tanks better then DPS.
**Note: This does not apply to everyone! Simply the general public.

Since people prefer the path of least resistance, most will swap to a new DPS class when they are bored of the primary job.