A few things.
- He came into power because the electorate were looking for change. We're still looking for it because he's just slowed the decay a little, and hasn't really made any big plays.
- Whereas leaders like Canada's Mark Carney have basically told Trump to take a flying leap off a short pier, Starmer has tried to play to his ego. He's invited him for a state visit, which is pretty much universally hated. He's negotiated over tariffs. He's normalised what Trump does.
- The government are currently fighting a court case where their lawyers claim the is "no evidence of genocide in Gaza" therefore arms sales to Israel are legal. Starmer is an ex-human-rights lawyer.
- He recently made a speech about immigration calling the UK an "Island of strangers". He's chasing a right-wing vote because the right-wing Reform Party are getting stronger. None of them would touch him with a barge pole so it's pointless. While he's doing this the left-wing base are ditching him for the Greens and LibDems.
So if you're a pro-austerity pro-israel anti-immigration Trumper who thinks Reform UK is a step too far, you'll be happy with Starmer. Otherwise you're looking for other options.