Nobody said people back there were stupid. I would say the contrary, even.
But that also means they were capable of making up propaganda fiction, tell lies, etc.
But on the other hand, they were not infallible super-geniuses either. And their audiences all super-geniuses either. In fact, Paul is kinda unkind about how smart or educated HIS audience was. Sometimes a novel is written with a certain audience in mind.
And on yet another hand, a lot of stuff was simply out of their control. They couldn't just recall all Bibles and issue a new corrected edition instead. It's not like it was a modern DRM-ed e-book that you can just revoke world-wide with a button.
The fact is, they actually tried to harmonize that stuff. There are several attempts, in fact at producing a harmonized gospel. But they never caught on, because, see above, it was out of their control. People revered the originals and were unwilling to replace everything with a mash-up story.
And various scribes did try to introduce changes that get the gospels more in line with each other. If you listen to Bart Ehrman, for example, it's actually one of the things he uses as a heuristic for which is the interpolated one between two versions of a text: people tended to make changes that brought the texts more in line with each other, rather than changes that create more conflict.
But again, things were out of their control. You couldn't force some other scribe to use your version, especially if it contradicted their favourite part.
Even for the original choice of texts, the more parsimonious assumption is that they just couldn't, rather than that there's some secret meaning in there. E.g., because each of the churches that Irenaeus helped unite into what would become the Catholic church, had another text they held sacred.
E.g., it's pretty clear that Irenaeus himself favours John for when it comes to arguing stuff based on gospel, while the others appear only when it's time to argue why it's those four gospels that count. Since he was the bishop of one of those churches. It's a safe guess that the church in Lugdunum (Lyon) was going by John.
Mark on the other hand, the general consensus is that it originated in Rome, and had a headstart on the others, so it's a safe bet that you couldn't just toss it away. Not if you wanted your church coalition to include probably the largest Christian population of any city at the time.
And so on.
There's no real reason to assume anything else than that they had no choice but to keep them. You could get a bunch of churches to unite and rationalize around the differences, but I doubt you could get any of them to just dump its holy text and use yours instead. Whatever church was using Matthew and its emphasis that the OT still matters, wasn't going to chuck it away and use Luke instead.