GFCI Did Not Trip - Outdoor Extension Cord Fell in Mud
GFCIs are not magic everything-fixers
It's like asking an ambulance crew &quot;why didn't you arrest those graffiti artists?&quot; &quot;That's not what we do&quot;.
GFCIs do one specific task that is related to human safety, and they do it well.  They detect current leakage between the intended hot-neutral loop and anywhere else.  However they do nothing about any fault that stays inside the hot-neutral loop.
Hence their name, &quot;Ground Fault Circuit Interruptor&quot;, which is ironic since they don't interact with ground at all.  It's presumed that the &quot;anywhere else&quot; must be ground.
Those other bailiwicks
It sounds like you had more of a &quot;poor connection/arcing/overheat&quot; problem.  That is more in the bailiwick of an AFCI.  (Arc Fault Circuit interruptor).  Those work by listening to the power line for that familiar &quot;crinkle-crunch&quot; sound of hooking up speakers with the amplifier turned on, or a shoddy headphone jack, etc.
You note also that the GFCI did not trip and the breaker did.  That's another thing GFCIs aren't.  Some people think if they put a 15A GFCI on a 20A circuit, they can extend from there with 15A wire and the GFCI will protect it. The opposite is true, 15A GFCIs are specifically rated for 20A pass-through. (because of a NEC technicality).