The TARDIS was supposed to change, to disguise itself, to blend in seamlessly with its varying surroundings.
It didn't.
Susan said the chameleon circuit was broken. The Doctor merely hrrumphed.
Whatever the reason, the TARDIS stayed looking like a police box; a blue painted wooden box, nine feet tall and four foot square: it was as much an anachronism as they were.
Barbara rather liked it.
It retained the shape it had taken to fit in with 1963. Regardless of the interior, it was a reminder, a souvenir of home.
She hoped the Doctor wouldn't fix the circuit too soon.