Easy. There is no party who stands out as being capable, or clear (and trustworthy) about their intentions.
Conservatives
Left the EU. Not only that but with a very bad deal (not that there was a better deal than the one we had as members).
Have had 14 years to implement some of the things they are promising now.
Have managed the economy badly (and then the Trussterfuck).
Have no clear leadership, and no heavy hitters in the wings.
Labour
Have no clear leadership or heavy hitters in the wings.
Starmer is very woolly. I don't really have a clue about who he is, or what he stands for. He is a typical lawyer in that he objects a lot, but doesn't really say what he would do. The manifesto makes a lot of promises, but is light on the detail of how those promises would be delivered.
Lib-Dem
Don't really have a chance of forming a government, but may be a good strategic vote to oust my local Conservative MP.
Reform
Not really worth dignifying with a comment.
I know that leaving the EU has caused economic problems that will take decades to solve, but there is no ownership of it within the Conservative Party. Couple that with the response to the pandemic, which was laughable, and we have a fairly dire situation with no plan from either party.
That's why.