I asked a similar question here a few days ago (though I'm after a 40") but no answers. Others have asked this question too so it seems there is demand for sub 55" screen sizes.
I suspect the answer is this: as long as the electronic giants can convince people to buy 55"+ screens, they needn't bother producing anything smaller. Chances are, their profit margins are larger with the larger screens.
AFAIK, only LG are currently manufacturing OLED screens (though other electronic manufacturers incorporate them into their sets) so only when there are not enough purchasers to consume LG's entire OLED screen output will LG likely consider making smaller screens. With successful marketing currently managing to convince enough folk to buy 55"+ screens, smaller OLED screens may take some time to reach the market - if they ever do.
If there were many electronics manufacturers producing OLED screens, I'm certain we would have a wider selection of screen sizes because the 55"+ market would saturate rapidly which would force the production of smaller screen sizes.
Promisingly, it seems there are several electronics giants currently working on the next generation of OLED screens, QLED. But then there were quite a few manufacturers working on OLED a few years ago.