I'm not defending the former POTUSES (POTII?), but 'tradition' holds that they don't speak out against the current office holder. This is supposedly to maintain 'political stability' or so as not to appear to be trying to usurp the current POTUS' authority. IMHO there comes a time when traditions no longer serve and this is one of those times. There are only 4 of them (Clinton, Bush, Obama, & Biden) and I have a feeling that we can count Bush and Biden 'out')
As far as Dubya, he made it clear when he left office that he was OUT of politics completely. And other than his (overheard) remark at Trump's 1st inaugural speech "That's some crazy shit right there" I don't think he's said anything. Frankly, I don't think he ever really wanted the job in the first place and was shocked when he found himself 'appointed' by the SCOTUS recount decision. I think Dubya thought it'd be an easy 4 years with governance being handled by Cheney and the GOP as the US economy was booming and we were basically 'at peace'. Then 9/11 happened and all Hell broke loose.
I think Biden is sick of politics too after the way he was slandered in 2024. And unfortunately that slandering has 'tarnished' him unfairly. Even his own party 'ran from him' after the disastrous debate. I'm not opining on his ability to continue to 'do the job', just that his 'cognition' was made out to be worse than it was, if it was 'bad' to begin with. I'm sure he doesn't want to open that PR can of worms again.
As far as Obama and Clinton, I don't know why they aren't speaking out more 'frankly'. Neither of them wants a future in elected office so have nothing to lose on that score. But both of them are doing a lot of 'good works' through their foundations so perhaps they're trying to keep out of the limelight to avoid those being drug through the mud.
All this is, of course, just my own musings.