People have mentioned the move from skill points to proficiencies. INT as a dump stat is really the result of this change. 4e calculated skills a little different than 5e, but neither of them used the skill points model.

In 3e INT was used for a lot more things, it affected how many languages you could speak, more skills needed INT, but it was used for generating skill points for all classes, which made it an important stat for skill classes like Bard and Rogue, and all spellcasters in general (because Arcana is a combination of multiple spellcasting/spellcraft skills). See this table:

[Linked Image from i.imgur.com]

So why is this important? People have talked before on this forum about MAD, Multiple Ability (score) Dependant. A class that is MAD has class-related dependencies on more than one ability score, which is in itself not a bad thing, but consider that on top of all your normal AS dependencies for your class abilities, you now also need a decent (or at least 0) INT mod so you aren't penalised on skill points. It was possible to end up being dependent on too many scores, and having to spread your ASI levels over too many abilities meant that they would all be lower than they needed to be. Your ability scores were NOT capped at 20 back then, and certain special feats or skill bonuses didn't unlock UNTIL some skills were above 20. If you're interested in what the actual skills were, here

4e and 5e tried to minimise MAD. Wizards wanted a simpler, more new-player-friendly system that was harder to screw up, and multiple INT-dependencies got chopped. CHA was always the main stat behind the second pillar of D&D, social interaction, that hasn't changed. And between 3e and now we only gained one more base class that uses CHA as it's spellcasting/class ability, which is Warlock (but we lost more base classes than we gained with the change from 3e to 4e).

On a personal note, I've always thought it would be interesting if Warlock was allowed to choose weather they wanted to be dependent on either INT or CHA. Choosing your spellcasting stat wouldn't work for most other classes, but I think with Warlock it would.