Roughly: Blockchains primary attribute is that they make everything public and attributed and it thus can be validated. Making everything public is the opposite of what we want to do with voting.
It's tricky to combine convincing people that their own votes were counted, with making people's votes anonymous, and with convincing people that only registered voters were able to cast votes (and only one vote each).
There is computer science research that tries to address this, but it's subtle, and sometimes people are concerned that voters would be unpersuaded that the math is right.
Two other general problems with online voting which are as hard, or even harder, with blockchain-based voting:
(1) If people are voting from their own devices, those devices might be hacked in order to make them submit votes that don't match the voter's intent. This might be an even trickier problem if the votes are being anonymized by a complicated cryptographic protocols.
(2) Someone might try to attack the election by interfering with the vote-collection infrastructure. This is probably easier on a public blockchain, where, for example, miners could try to prioritize blocks that don't contain information about votes, or people might try to do fork attacks specifically during the election to cause fluctuations in estimates of whether votes were included on the chain or not.
Some of these things can potentially be addressed with even more cryptography, but I think the biggest insight is that just having a public ledger won't itself convince people that only registered voters were able to cast votes (and only one vote each), especially when the votes are recorded anonymously. In fact, just having a public ledger won't even interact with this challenge!
(Debian has had a clever solution to some of these problems for a while, just using a digitally signed web page rather than a blockchain, but their voters -- Debian developers -- are technologically sophisticated and publicly listed in a database that all other voters can agree on.)
Also could we anonymize the voter that way it’s like any other legally private information that can be accessed with a warrant etc.
https://en.wikipedia.org/wiki/Secret_ballot
https://en.wikipedia.org/wiki/Preference_falsification
https://en.wikipedia.org/wiki/Social-desirability_bias
(I think Brendan Eich's experience with suffering enormous professional consequences for having donated to the Proposition 8 campaign -- a fact which was public under California electoral laws -- is a concrete example. You could imagine the same kind of consequences for everyone who voted for the proposition, in which case many people would not only choose their political donations based on fear of public opinion and social and professional consequences, but would even choose their votes based on those same fears.)
These issues might not convince you, but they represent a sample of other people's answers to your question. Also, in many places, depending a lot on the culture and history, a powerful local figure such as a large employer, union leader, "patron", or religious leader will actively try to tell people how to vote, if given the opportunity.
https://en.wikipedia.org/wiki/Political_machine
https://en.wikipedia.org/wiki/Electoral_fraud#Vote_buying
[1]: https://www.cnbc.com/2020/10/28/2020-election-spending-to-hi...
Voters could be rewarded for voting a certain way.
Voters could be punished for voting a certain way.
You can't get rid of rewards without eliminating capitalism, and you can't get rid of retaliation without eliminating power structures.
Making voter information anonymous except via subpoena doesn't work for the same reason that backdoored encryption doesn't work.