As a society we have decided that "secret ballots" are better than "open ballots". When you use a blockchain you will issue public keys to individuals who can then verify their votes were counted. I'm aware that the voter's name is not on these keys (I could not look at the chain and determine how you voted), but regardless it is a way to prove how you voted (I can prove to you how I voted). This means that you could do things like sell your vote or people could try to intimidate you to vote certain ways.
I'm not saying this is a good or bad idea, since in the past "open ballots" had something more explicit like your name attached, but this is a reason why people may be hesitant to use blockchain.
We need to decide which is more important,
- ensuring a way for the voter to check that their vote was accurate and counted to combat corruption by altering counts
- or ensuring secret ballots to combat corruption by people coercing or forcing people to vote certain ways
One great point Maeher mentioned in the comments is that with paper ballots the tradeoff of not being able to verify your vote is correct and counted when using paper ballots does not necessarily need to exist. If you view the entire process from putting your ballot into the box to the counting of the ballots then you can know for certain your vote was counted. Then make sure that the numbers reported during the counting match what is publicly reported from your place of voting.
The above is more of a philosophical why a blockchain wouldn't work. You also mentioned "I couldn't find any proof that the e-voting is definitely insecure." I can comment on that as well. Not all electronic voting would involve a blockchain, so you can have secret ballots while still using electronic voting.
How do you know what software any kind of electronic voting system is using? You simply cannot. You may suggest something like getting a hash of the program, but how do you know the device performing the hash has not been tampered with and how do you know that the device being hashed isn't trying to fake something? You really just can't know.
Paper ballots on the other hand are extremely easy to reason about. Physical things are just simpler. You don't need to worry about if the terminals are secure or recording properly, you simply know your ballot has the right vote because you can see it. Then the ballots will be loaded into boxes and counted somewhere. Parties from all sides of the election will be present to ensure nothing fishy is happening, and they will be able to spot it even if they are a non-technical person.
So it's not that electronic voting is "definitely insecure", it's that it's harder to know if it is.