Teacups are not suitable for everyone's lifestyle.  In most cases, someone wanting a teacup really wants a tiny 4-5 pound maltese.  These tiny babies will fit  nicely in your lap or in the popular papoose back  packs or pet pockets when they are full grown but will not require the extra care that a teacup will require.  Teacups can burn up their energy stores quickly (as there isn't much there to begin with) so they have to eat frequently throughout the day and be monitored to make sure they are actually eating or they can end up in a hypoglycemic~low blood sugar~ attack or coma.
Teacups require frequent meals, balanced by lots of rest, but can make adorable pets for someone who has all day, every day to spend with their teacup baby.