The markets want to know that the UK can pay off it's debt. The government announced a massive spending package along with massive cuts to tax revenue. As well as this, they did it without the normal independent analysis on the impact, and have signalled they don't think they need to worry about the UK balancing it's budget in the short term. So the markets don't think the UK will be able to pay off its debt because the government announced plans that will cause it not to be able to pay its debt. Simple as that.
Normally left wing governments are punished because they increase spending without increasing tax revenue - causing unsustainable debt. The UK has done the right wing equivalent - they decreased tax revenue without decreasing spending - causing unsustainable debt.
You're starting with an assumption that right-wing governments are good for economies. Let's ignore that dubious assumption and instead look at the actual policy.
Truss's administration is giving tax cuts to the wealthy and paying for it by borrowing. The markets know that trickle-down economics is nonsense and these tax cuts won't boost UK's economy enough to cover the enormous amount of debt required to pay for them.
It's probably not this. Part of the issue here is that markets do believe this will boost demand in the UK economy. It's just that this isn't good right now with inflation so hot. If anything you want to be increasing tax to lower demand.
As an investor you always want your investments to yield more than the rate of inflation and when government policy increases inflationary pressures by fuelling growth the market will demand higher interest rates. Of course there is also uncertainty about how the government will pay for the debt which is increasing the risk premium.
It's just dumb fiscal policy to boost growth at a time when central bankers are trying to do the opposite. It would probably work great during a deflationary period. Its just that's not where we are right now.
"Good for economies" does not mean much, and that's what distinguishes the left and the right.
Usually, the right-wing, and the "markets", agree on what is good for the economy. It is quite unique to see the "markets" disagree so strongly with "the right".
And it is also worth noting that it is quite rare to see left people use the view of Wall Street as an argument ! I remember many cases in Europe where socialists governement almost used it as an electoral argument that they would ignore "the markets" (or rating agencies) and not dictated their policy by what they said
That sort of fantastical thinking is quite rare outside of fringe parties (left and right). The UK Labour party's spending plans in 2019 were fully costed exactly in order not to spook the markets. (Naturally that's not how the media portrayed the situation of course.)
Ignoring the markets hasn't been possible since Nixon ended the gold peg.
Markets don't care about politics, they care about risk and reward.
By fuelling growth in a period of high inflation with debt you basically add to the existing inflationary pressures and increase risk.
As an investor you want your investment to at least yield more than inflation so this is one reason why the market is demanding a higher interest rate (the market now excepts UK inflation could run hotter), but they're also worried about the risk of extra borrowing so are also demanding a higher risk premium because of the uncertainty of their investment.
Also, I'd argue this isn't really a "right-wing" fiscal policy. Cutting taxes might be "right-wing" but borrowing to do so certainly isn't something that's universally popular with those on the right. Most on the right probably want tax cuts to be funded by spending cuts.
The counter factual where a left wing government does something this stupid is hard to imagine because the press and markets normally go so apeshit at even minor transgressions of neo liberal orthodoxy that we never get that far.
Normally left wing governments are punished because they increase spending without increasing tax revenue - causing unsustainable debt. The UK has done the right wing equivalent - they decreased tax revenue without decreasing spending - causing unsustainable debt.