At least in the USA you have CNN and, to an extent. MSNBC to counter the Fox diet of MAGA and Trump's latest bombast, whereas here there is nothing mainstream to offer an alternative to GB News. The BBC is gripped by its impartiality 'rules' now being interpreted to mean that the Tories and their many leaders, ex-leaders and nonentities like Rees-Mogg must have their pronouncements treated as though they come direct from God. Johnson and his advisers are now very much playing Trump cards that any criticism or sanction is outrageous and biased.
There is no mainstream alternative to GB News - it is not, by their own words, a news channel. Their programming is based around opinion, while what is portrayed as their competition are all based around factual news - with a clearly defined line between what is and isn't opinion (e.g. you don't get newsreaders giving their opinion on the days news, they go to third parties for that - and on the odd occasion when a newsreader does veer into opinion, they get told off and apologies are thrown around like confetti).
GB News do not want to be a news channel - otherwise they would fall into the same impartiality difficulties that are faced by the likes of BBC, Sky and ITN in their programming. All GB News have to do is show that there is a range of opinions - be that with a token gesture of someone the presenter disagrees with given a few moments to present an alternative - and they have fulfilled their requirements under the broadcasting code (which is where Mark Steyn fell foul, because he presented his interpretation of a Covid data set as being
the interpretation - if he'd just had a single sentence of "of course, there are alternative interpretations, and we don't know what sort of uptake there has been of the third vaccine" and he wouldn't have been censured). Notably, the requirement for alternative opinions to be visible is not present in any US broadcasting rules, so non-UK contributors may not be aware of (or agree with) this requirement.
The initial airings of GB News sailed very close to not having sufficient separation of opinion and factual presentation, discussions of which (and their need to be seen as not a news channel) is what brought about the short news bulletins with a different presenter. Laughably, they originally tried to say the weather bulletins were the news output, while everything else was opinion, but that flew with nobody.
As to why there isn't an alternative to GB News leading on centrist / leftist opinion - well I would propose that it's mostly that nobody really cares. There seems to be a part of the right that really thrives on hearing their opinion reflected back at themselves, and they are a big enough commercial market to make it worthwhile to produce programming for. I think there is a similar part on the left, but they're far less trusting of the commercial world and are less likely to spend money on things they've seen advertised - there is a thriving left wing community publishing industry, which every time one of those publications seems to start getting more prominent, collapses under cries of selling out or going mainstream.