Well, the problem is that if bridge is discovered by child pornographers and used by them - police will get knocking to bridge owner's door.
No one needs that, hence the supply is low.
No one needs that, hence the supply is low.
Edit: http://arstechnica.com/tech-policy/2012/11/tor-operator-char...
+-----+
| You |
+-----+
\ <= encrypted _
+----------------+ | T
| Tor Entry Node | | O
+----------------+ | R
\ <= encrypted |
+------------------------+ | N
| Tor Bridge/Relay Nodes | | E
+------------------------+ | T
\ <= encrypted | W
+---------------+ | O
| Tor Exit Node | | R
+---------------+ _| K
unencrypted => \
+----------+
| Internet |
+----------+