It's all about the power handling.
Jacks have a current capacity of around 5 Amps, which equates to 100W @ 4 Ohms, 200W @ 8 Ohms and 400W @ 16 Ohms. That was all well and good when the biggest amps were 200W Marshall Majors, but times have changed.
There was a trend in the late 70's / early 80's to use XLR connectors for high power speaker connections. Most 3-pin XLR's are rated around 15 Amps, which gives 900W @ 4 Ohms, 1800W @ 8 Ohms (we can ignore 16 Ohms from this point on, as it's only valve amps that output into 16 Ohms)
Speakons can handle currents of up to 30 Amps, in fact the new 4-pole versions are rated for 50. 30 Amps equates to 1800W @ 2 Ohms, 3600W @ 4 Ohms and a ridiculous 7200W @ 8 Ohms.
With modern bass amps commonly putting out upwards of 500W into 4 Ohms (and let's not even get into power amps, many of which can deliver over 2000W in bridge mode, why would you use a jack? In 1963 it was the best that was available, but since then the rest of your rig has moved on, so why not the speaker connectors?
A