as noted, it's to celebrate the 60th anniversary of the series. with plans to create a full website containing various archived materials alongside it.
they've also notably gone through each episode to add full subtitles, audio description, and sign language(!) options for accessibility purposes.
the reason why it's not everything is thanks to the BBC rather infamously having no archival team until 1978, and frequently wiping master tapes to use them on later shows. the amount of effort it took just to get the earlier episodes we have is truly worthy of it's own thread.
as for plans outside the UK, Disney+ currently has mostly worldwide rights for new episodes of the Doctor Who series starting in November. while the pre-2005 and post-2005 (until November 2023) episodes are split between Britbox and (HBO) Max respectively, at least in the US.
making it unlikely a similar deal will happen for a while.
editor's note: while making this thread, someone decided to create an ST for the series. as this is a proper news story rather than a general discussion thead, i'd assume both threads should be able to exist without issue.