It has to do with a combination of niche protection combined with some mechanical differences that went away. Here's thre step by step of how the entire niche was erased in a shift from niche to default:I still think I'm not getting it? Or at least how your explanation of Knowledge skills is making INT less desirable? The condensation of dozens of INT-based Knowledge subskills down to 4 "super-skills" doesn't reduce the number of times INT is used or asked for... it just reduces the number of proficiencies that are required to get better at any of those things.
All those subskills you mention all still use INT as their baseline stat... regardless of whether or not the check asked for is the subskill itself, or the 5E skill that it falls under. If you ask players for a check about the Migration of X tribe... in 3.5E and 5E they both will add your INT mod to the check. In 5E it'd be an INT (History) check... in 3E a Knowledge (Migration) check that includes the INT modifier in it. In all cases... the checks made will include the INT modifier. That number doesn't change. The only thing that changes is the other bonus amount from skill points or proficiency bonus.
Now granted... there are several skills in 3E that use INT specifically and do not necessarily have a corresponding INT-based check in 5E. Appraise in 3E uses INT... and in 5E the DM would have to choose for themselves which ability score would apply (probably INT, but it doesn't necessarily have to be). Disable Device in 3E uses INT, whereas in 5E it would probably be DEX (Thieves Tools). Forgery in 3E uses INT, but in 5E again it would be up to the DM to choose which ability score you'd add the proficiency in Forgery Kit to. Probably INT again... but not necessarily. And finally anything Spellcraft related in 3E would fall under Arcana in 5E and thus use INT as well. So all told 5E lost INT use in disarming traps (which COULD be a big deal and number depending on campaign) and there might be a few other occasional checks lost to INT in 5E... but those seem few and far between to me.
I do not disagree that 3E uses INT for more different things than 5E-- the additional skill points gained for high INT being the main thing-- but in terms of the Knowledge skill / subskill condensing? That doesn't affect the use of INT at all as far as I can tell. They are all INT checks and the only different is the proficiency/skill point bonus you add to it.
- If you did not have points in a knowledge skill it was considered untrained.
- With knowledge skills :untrained" meant you could only ever get DC10 info & lower
- Anyone could be considered "trained" by putting one rank in a skill (ie knowledge:nature-vermin not knowledge nature-*) but if the particular skill was not considered as class skill doing that cost two skill points rather per rank than just one
- bardknowledge any
- Bards had 6+int mod skill points but int was not a big attrib for them & they tended to be incentivized to prioritize social skills & such
- ClericKnowledge (arcana) (Int), Knowledge (history) (Int), Knowledge (religion) (Int), Knowledge (the planes)
- Cleric domains gave a little extra based on the domain , but with 2+int mod skill points/level knowledge skills beyond cleric specific stuff were not too high on their list unless needed for a PrC or feat choice
- Druid: Knowledge (nature)
- Oneknowledge subtype (nature) & withg 4+int mod there were reasons for them to not put too much beyond a point in a couple of the jucier five nature knowledge types
- Ranger: Knowledge (dungeoneering) (Int), Knowledge (geography) (Int), Knowledge (nature)
- dungeoneering was aberrations, caverns, oozes, & spelunking but with 6+int mod there were a lot of other more important skills to sink points into
- Monk: Knowledge (arcana) (Int), Knowledge (religion)
- 4+int modskill points & the same other skills they want for mechanical reasons
- Paladin: Knowledge (nobility and royalty) (Int), Knowledge (religion)
- Rogue: Knowledge (local)
- 8+int mod but tons of skills they want more for mechanical reasons
- Sorcerer: Knowledge (arcana)
- with 2+int mod & possibly a 6 in int they almost certainly have better places to stick skill points
- Wizard: Knowledge any
- 2+int mod but having a prime attribute climb above 20 was not uncommon for this to be +8 to +10 or more points/level. They also had a very short (but nonzero*) list of skills they felt were mechanically important & could afford to splurge a point or two here & there as they advance
- Fighter/barbarian: N/A
- bardknowledge any
- As noted int mod added to skill points gained each level so adding just 1 point to your knowledge skills made a big difference if you invested in intelligence but because there were a bunch of subskills it was not a trivial choice for most who could & not painless to go beyond the first point to be considered "trained".
- 5e both condensed the knowledge skills & made them easy to take without pain. If a GM makes a particular theme of knowledge checks important & has Alice with a know stuff class while Bob Cimdy Dave & Edith have classes with no reason to know that theme it's trivial for them to make Alice's moment into "I want to try too, what do I know?" just by taking the obviously campaign relevant skill
When the wizard has +5 +6 or whatever to all of those & you couldn't get more than dc10 stuff without being trained in the knowledge subtype it mattered a lot. Sure the wizard would probably want the druid/ranger to do nature: vermin or something because they probably only had a single point in it before their int mod & maybe a tool/skill book brought it up.
Now if I call for an knowledge check in 5e it's not uncommon to have more players with the relevant skill than not unless it's a rather niche one. Worse than that, I simply can't use the niche skills for stuff because they all probably have arcana or religion instead of ones relevant to their class' thematic niche in addition to all of the skills they have a mechanical reason to want for that niche.
*Ironically a big chunk of those skills (spellcraft, decipher script, & concentration) are gone or just a con save now.