Neil Williams wrote:

As to why bus stations have fallen out of favour, I don't know. In
Milton Keynes' case there is a good reason in that the centre is so
dispersed that all buses run the length of it on a single route (which
is useful in itself), and it was decided that stopping in front of the
railway station was more sensible than in the bus station which is a
short walk away.

One of cost maybe. In the case of a local authority owned bus station
there will be a charge of so much a departure whereas using a nearby
street will be free - or at least cheaper. If the bus company owns it
the temptation to sell it off for development must be high.