Having our First Past the Post electoral system forces political parties to be very 'broad churches' in order to have widespread support across enough of the UK to form a majority. Within the major parties; there is a broad span of opinion and multiple potential areas for fracture and disagreement. Elections tend to be won in the centre; and Corbyn was perceived to be left or far left rather than centre left. The Conservatives are going to have problems after the election in working out whether they want to be centre-right or more further along on the right.
Our whole system would be better if we ditched FPTP and enabled there to be more ideas and debate; with people having better choices on which political parties have ideas that most closely resemble their own.
The Corbyn/ Starmer relationship is particularly challenging due to allegations of antisemitism within the party when he was leader
In 2020 The Labour Party was served with an unlawful act notice after an investigation into antisemitism by the Equality and Human Rights Commission found it responsible for unlawful acts of harassment and discrimination.
The report didn't lay blame directly with Corbyn but the EHRC’s lead investigator, Alasdair Henderson, said the failure of leadership must ultimately stop with him.
“As the leader of the party at the time, and given the extent of the failings we found in the political interference within the leader of the opposition’s office, Jeremy Corbyn is ultimately accountable and responsible for what happened at that time,” https://www.theguardian.com/politics/2020/oct/29/jeremy-corbyn-rejects-findings-of-report-on-antisemitism-in-labour
Corbyn rejected the findings of the report. Starmer is a previous Director of Public Prosecution; and having a finding by the EHRC that the party has broken the law is signifcant. Starmer was serving in Corbyn's shadow cabinet at the time of the election. Labour eventually expelled Corbyn from the party, and he is standing as an independent within the constituency that he formally held for Labour.