I have tried looking at this from both perspectives and can see a number of different views - however, I keep coming back to the same point.

The space's and easy access vehicles, came about due to legislation and pressure from disability groups.
Bus operators and manufacturers, were left with the costs, for what essentially is a small portion of their market (how many wheelchair users do you see?).

The changes to bus layout and design, reduced seating capacity and also space to store bags, shopping and folded bulky items, such as pushchairs.
Sensing a marketing opportunity, the operators promoted the space created for wheelchairs, advertising its suitability for pushchairs.

Excellent, except that able bodied users view the seats as their own, often refusing to move for pushchairs.
Those with pushchairs, have refused to move for wheelchairs.

You also get the complaints made by those with pushchairs wanting to get on a bus, but unable to due to a pushchair or wheelchair already being on board.

Which brings me back to the point about easy access vehicles and lack of storage facilities/luggage bays.
There isn't anywhere to store a folded pushchair - due to the design of the easy access vehicles.

Whilst not apportioning blame, wheelchair users have been put in this position, by virtue of the successful (as it should have been) campaign to make buses accessible to all.