I became interested in crypto lately and read about symmetric and public key crypto algorithms. I understand how crucial the discoveries of the 1970s like RSA, DES and DH were in advancing the technology.

I just don't understand how public key cryptography as an idea just came less than 50 years ago while it was a highly desired solution throughout history and kingdoms/countries/intelligence services would pay handsomely to whoever discover this simple yet revolutionary idea.

Things like integer factorization and elliptic curves were studied since the 19th century, how come nobody thought of using such known mathematical problems to create strong crypto systems, especially in WW1 and WW2 where confidentiality and integrity of data could decide the fate of a whole nation, instead of relying on centuries-old futile crypto techniques?

I am sure that this wasn't a lack of human resources, every European country had lots of geniuses, so why  wasn't public key cryptography discovered until so late?

If you read about Merkle's experience with bringing the subject to the mainstream, you might be surprised. His professor rejected his initial proposal and show little interest in any further developments:

...We were required to submit two project proposals, one of which we would complete for the course. I submitted a proposal for what would eventually become known as Public Key Cryptography -- which Hoffman rejected. I dropped the course, but kept working on the idea.

He showed it to a different faculty member and was encouraged to publish it. His proposal to the Communications of the ACM was rejected:

... I showed an early draft to Bob Fabry, then on the faculty at Berkeley, who immediately recognized it as both novel and valuable and said "Publish it, win fame and fortune!" I then submitted it to Susan Graham, then an Editor at the Communications of the ACM in August of 1975. As I was to learn, Fabry's response was rare.
Graham sent my submitted paper out for review and received the following response from an "experienced cryptography expert" whose identity is unknown to this day:
"I am sorry to have to inform you that the paper is not in the main stream of present cryptography thinking and I would not recommend that it be published in the Communications of the ACM."
"Experience shows that it is extremely dangerous to transmit key information in the clear."
With this blanket rejection of public key cryptography by an "expert", she rejected my article. She "was particularly bothered by the fact that there are no references to the literature. Has anyone else ever investigated this approach. If they consider it and reject it, why?"

Novel concepts that aren't "mainstream" can be difficult to accept:

I had failed to provide any references to the prior work on public key cryptography, and the reasons previous workers in the field had rejected it as impossible. I should have looked up "public key cryptography" on Google before submitting my paper. My defense is feeble: there was no Google, the term "public key cryptography" did not yet exist, and there were no previous workers in the field. There were no words for what I had done, and looking up a concept to show that no one had previously thought of it is difficult. This is not a unique problem: it illustrates a problem faced by anyone trying to explain a new idea to an "expert" who expects a properly referenced article anytime anyone tries to explain something to them. The more a new idea is unrelated to any prior idea or concept the more it must appear as a squawling bastard, naked and alone, appearing de novo and lacking any respectable pedigree or family to vouch for its acceptability.
... CACM eventually published the paper, though only after almost three years of delay, and only after others (who were better able to persuade their editors to publish in a timely fashion).

Conclusion
While the utility of public-key cryptography is obvious to us, this was not always the case. Encryption that is secure when you know the key almost sounds like something that can't exist because it would be too good to be true. So you can certainly forgive people for not pursuing the idea seriously.
