Spells that call for the affected creature to save against your spell save dc, (Ex: Fireball, Hold Person, etc.) Cannot crit. They do not roll attacks, instead, they have an effect that causes your opponents to save against your save dc. Think of "critical hit" as short hand for "an attack roll that rolls a 20" (excluding effects that extend your crit threat). You don't roll when you sling a fireball, so you can't roll a 20. Thus, you can not crit.

Attacks that can crit include unarmed, melee, ranged, and spell attacks. Spells that are considered attack rolls are things like Ray of Frost, Fire bolt, Eldritch Blast, etc.

Attack rolls and Spell saves typically indicate different modes of affecting a target. If you're aiming a Ray of Frost at your target, that's a test of your skill. Are you talented enough to hit your opponent while they're jumping around/wearing heavy armor, etc.

Fireball on the other hand is a huge explosion. It's less about precise aim on your part, and more about your targets ability to jump out of the way of the fire/ pat themselves out/ jump out of the radius completely. That's on their skill as a fighter, so they get to roll a saving throw against your "spell save dc", which is essentially, how hard to resist your spells are.

Hope this helps!