Thai Lion Air primarily flies short-haul domestic routes and most of the time I find myself on an ER version of a relatively new 737. Isn't ER for extended range? And thus costs a lot more per plane?
Incidentally their sister company Malindo also has what seems to be an unnecessary amount of ERs (I just flew on one this morning from KUL to BKK - a 110min flight - and one the week before from KUL to HKT, again under 2hrs).