Couple of factors. Firstly, that Pat Mills wanted it to be very loud and crazy and in your face, but knew from the censorship/closing of his previous series, ACTION, that he had to hide it a bit under sci-fi. Which allowed things to go weirder and more out there.
Also, they knew that they were aiming at kids aged 6-10 more or less, so the stories especially in the early years are very bombastic and loud, but over time you can see them grow in complexity and depth. 2000AD in general never looked down on kids, and as the audience grew, the writers saw the chance to grow the strip and took it.
And of course, the writers and artists were a fucking murderers row of British talent, most of them veterans from years of work before they even started in 2000AD. Wagner and Mills got their start writing girls comics, which Mills in particular credits with how they created characters.
It's honestly really interesting to read up on all that. Mills and Steve MacManus both have books on the subject that I can definitely recommend.