I should note that critical hits are fundamentally different for ranged and melee weapons. Ranged weapons have a much lower base chance (2%), which scales up by 1% for every 80 damage dealt in the last 20 seconds. This is damage from any weapon or sentry gun, not just from the weapon itself. It caps out at 12%, which is per-shot on most weapons and per-second on fast-firing ones like the Minigun.

Valve has previously stated that they included critical hits in this way to enhance the “high” of a winning streak, giving high-scoring players a little extra fun before their inevitable demise. While critical hits are extremely contentious, this isn’t the place for that discussion; we’re talking about the “no random critical hits” modifier.

On ranged weapons, it’s usually included when they have some other method of scoring guaranteed crits - think the Phlogistinator or the Frontier Justice. In all honesty, I can’t find a good, solid reason to eliminate random crits on these weapons, but I can speculate.

The main point is that a guaranteed crit provides a similar kind of “high” as a random one does, only you can control when it strikes. If critical hits represent power surges for the player scoring them, it’s fair to make them come in only one particular way. Since ranged weapons score crits relatively infrequently, the attribute doesn’t balance things out all on its own, but serves as kind of a way of modulating gameplay highs/lows. That’s my best guess, and I’ll freely admit I can’t say for sure.

Now, on to melee crits. They work differently than ranged crits simply by having a higher chance. They start at 15% - already higher than the highest you can get on a ranged weapon - and scale up by roughly 1% for every 18 damage dealt. They cap out at a whopping 60% crit chance - not the best betting odds, but enough to make any opponent think twice about approaching.

This means that disabling random melee crits is a much more significant downside than disabling ranged crits. Melee weapons with random crit potential always threaten to take the opponent by surprise, defying the maxim about bringing a knife to a gunfight. That’s a pretty good reason to deny that random ability to weapons which can already score guaranteed criticals. 

In very basic terms, some weapons tell the enemy when they can crit, but always do so in that case; some weapons don’t tell the enemy, but don’t tell the user either. It’s not a perfect system, as evidenced by strong opinions toward critical hits, but this is my best explanation. Maybe I’m completely off my rocker, but this is the rationale I use when adding “No random critical hits” to a weapon.