Q:

Why, from a lore perspective, are druids the only class who can cast the Flame Blade spell?

I am only really familiar with D&D 5e, and whatever of D&D 3.5e I was exposed to whilst playing NWN2 some number of years back, but as far as I am aware, only druids have the spell flame blade on their spell list.
At face value, I can't see why this would be. Summoning a sword of fire seems like the sort of thing that sorcerers or warlocks should be able to do as well; in 5e at least, they can cast shadow blade, so why not flame blade?
Originally, I thought I wanted to know the designer-reasons behind this decision (and hence didn't ask it, as that's off topic), but then I realised that if such a designer's reason didn't include anything about the in-universe lore behind it, then I would be left rather dissatisfied (i.e. if it turns out that it's because, I don't know... Gary Gygax lost a bet back in the day, and that's the official reason, then technically my question would be answered, but that's not really what I'm looking for).
What I'm really after is why this makes sense from a lore perspective. Is there something special about forming a blade out of the element of fire that is specific to druids, or requires nature-y divine magic to perform? I accept that it is the case that, RAW, only druids can cast flame blade, but why does that make sense in-universe? If my sorcerer, the head of the Sorcerer's Guild, saw a fellow party member, a druid, cast flame blade, then my sorcerer may well wonder "why can't I, nor any sorcerer I've ever met, do that?". That's what I'm after.
So my questions are:

Firstly, is my premise actually true? Across the editions of D&D (for which flame blade exists), has it always been a druid-exclusive spell? (Note that I'm excluding the ability to cast it as a racial spell, such as the Mephistopheles variant tiefling from Mordenkainen's Tome of Foes, and I'm also excluding bards learning it via Magical Secrets and other such hacks; this is about whether or not it's on a classes' default spell list);
Secondly, the title question: why, from a lore perspective, are druids the only class who can cast flame blade? I'm looking for answers with citations from any official material from any edition of D&D.

If the answer is dependant on a specific setting, let's assume that the setting is the Forgotten Realms.

A:

It comes from the druid's connection to nature, and was originally a druid spell.
Flame blade actually first appeared in Dragon #71 (March 1983), in the article New druid spells... naturally!, by Gary Gygax.
In that article, it was originally a druid-only spell. The material components were mistletoe and a leaf of sumac, and the blade was wielded as a scimitar, the druid's traditional weapon.
The druid's association with the scimitar was in turn was based on its similarity in appearance to the curved sickle, which druids were thought to have used to harvest mistletoe or other plants. The druid needed a proper one-handed weapon and the English straight longsword didn't seem to fit thematically for a Celtic druid of pre-Roman Britain, so the scimitar became took its place. This is reflected in D&D lore: The Complete Druid's Handbook asserts that the curved scimitar represents the crescent moon.
Given the other spells in this article—detect poison, precipitation and goodberry, to name a few—it seems most likely that flame blade is a druid spell because fire is part of nature. The other spells in that article all relate to nature (e.g. water, sunlight, plants), and the druid's connection to nature in D&D lore is well-established in the class' description.
AD&D 2e reorganized all classes into one of four traditional archetypes, which made flame blade a Priest spell of the Fire sphere, opening the spell up to the cleric class as well as the druid. This isn't an unthinkable addition, as there are biblical angels described as wielding a sword of fire, so it's thematically reasonable. However, the cleric in AD&D 2e is forbidden from wielding edged weapons, which would preclude proficiency in the sword.
In AD&D 2e you also see the druid with specific connection to the four elemental planes. At 18th level the druid can enter the Elemental Plane of Fire, and by level 20 can enter all four Elemental Planes at will.
The druid's connection with fire is also well-attested in AD&D sources. According to The Complete Druid's Handbook, druids hold festivals which build bonfires to ward off evil. A druid is also described as using fire to enact revenge against a wizard using fireball in a sacred grove. Druids also gain a +2 bonus to save vs fire and electricity. They have other fire spells, such as wall of fire, conjure fire elemental, fire seeds, protection from fire, and so on.
All this said, the exact lore reason about this exact spell is essentially just a rationalization for the real-world reason, which is that Gary Gygax was inventing new druid spells and thought a flaming sword suited the class.

