Why are there so many different types of screws (phillps/flat/hex/star/etc)?
Shamelessly ripped off from here, looks like the primary reason why we still have "bad" screws (flat head and Phillips) is that the "better" types of screws are simply more difficult/expensive to manufacture:


  The reason for the different styles is
  cost and torque.
  
  The slotted head screws are cheap and
  easy to make. But they're completely
  useless for powered screwdrivers and
  you can't put much torque on the screw
  without it either slipping out or
  stripping the head (and maring the
  surface of whatever you're screwing).
  [Mad]
  
  Phillips screws are self-centering,
  making powered screwdrivers possible.
  They're somewhat more expensive to
  produce than slotted-head. They tend
  to 'cam-out' easily under torque,
  making it hard to apply much torque.
  I've heard they were designed that way
  to prevent overtightning. However,
  it's not good for exposed fasteners to
  look stripped.
  
  Robertson-head and allen-head
  fasteners can handle more torque than
  phillips-head fasteners, but are more
  expensive. Because the bottom of the
  hole is flat (unlike the pointed end
  of the phillips), there's more contact
  area and so it's less likely to
  cam-out. The robertson-head is cheaper
  than the allen-head, but the
  allen-head has six points of contact
  rather than 4, making it less prone to
  rounding out the hole.
  
  The Torx-head fasteners solve the
  problem of rounding/stripping by
  having the flat bottom of the
  robertson/allen that reduces cam-out,
  but it has much better contact with
  the driving bit to prevent stripping
  the head. The points of the 'star' on
  the driving bit engage the recesses on
  the screw at nearly right angles, so
  it has a very positive contact. Torx
  is becoming more and more popular
  because of that, particularly in
  assembly-line work.
  
  Because they're less likely than a
  phillips to be damaged when
  tightening, the allen (internal hex)
  heads are often used for exposed
  ('decorative') fasteners on 'some
  assembly required' furniture. It's
  also very cheap to make the allen
  keys, so they usually include one with
  the fasteners.