The original style saddles used from 76-93ish have vary narrow grooves for the string to sit in. When the bridge was a string through, there was no issue with strings getting pulled out of the string slots. Then when the bridge got changed to a top-load bridge, some heavy handed people would tend to pull the string up out of the slots playing too hard because the the angle the strings came from the tail piece and over the saddles (has never been an issue with my old MM's). Flea had this issue, so EB changed the saddles so there was no more side angle of the string coming from the tail piece, the string were now straight out of the tail piece and had a better break angle over the saddles, no more issues of them getting pulled out of the string slots.
Then in the late 90's EB changed to the short style bridge and put those massive bridge saddles with deep slots we see today.
You can see pics and history at Link Removed