[PeterisP]

Because they are unable to multiply on their own.

A weird bacteria or nanomachine might convert everything to other weird bacteria or nanomachines. A weird virus can only convert all cells to weird viruses, since it needs cells to multiply. A weird prion can only convert other prions to their form, which is even more limited. Both of the latter scenarios can be very dangerous, up to an extinction event, but they're different from gray goo.