"x1 connectors which doesn't have an "open back" for larger cards..."
Because they bothered to read the PCIe spec.
Open back slots are against the PCIe specification ("up plugging" is the term IIRC). That is for two (three?) reasons, first its a mechanical thing, if you noticed plugging a big heavy x16 GPU into a x1 slot is an exercise in getting it just right. Any bump/whatever will move the card into a bad angle, even if its screwed into the back panel. Second, in the past, with older pcie specs the amount of current a slot was required to supply was dependent on its size. So a big x16 card might need the full slot rating while a smaller slot might only provide a smaller amount of current. (IIRC the limits were something like 75W for a x16 and only 25W for a x1). Thirdly, down plugging _IS_ part of the spec and the correct way to handle this situation. That means the MB vendor provides a larger, say x16, mechanical slot which is provides a fraction of the signal lanes, say x4.
So, the vendors which were providing cut back slots were either ignorant of the spec, too cheap to pay the couple extra cents for the larger connector, or had some fundamental constrain on providing it and were willing to provide a non-conformant part (generally unlikely). I've seen these cut back slots a few times in parts of the arm ecosystem where the vendor can't be bothered to read the spec much less implement it correctly. This is also how one has the pile of problems that are frequently seen in the SBC market where the boards won't actually work with some PCIe card, USB device, whatever because the HW/SW is only implementing the convenient parts the relevant standard.
Because they bothered to read the PCIe spec.
Open back slots are against the PCIe specification ("up plugging" is the term IIRC). That is for two (three?) reasons, first its a mechanical thing, if you noticed plugging a big heavy x16 GPU into a x1 slot is an exercise in getting it just right. Any bump/whatever will move the card into a bad angle, even if its screwed into the back panel. Second, in the past, with older pcie specs the amount of current a slot was required to supply was dependent on its size. So a big x16 card might need the full slot rating while a smaller slot might only provide a smaller amount of current. (IIRC the limits were something like 75W for a x16 and only 25W for a x1). Thirdly, down plugging _IS_ part of the spec and the correct way to handle this situation. That means the MB vendor provides a larger, say x16, mechanical slot which is provides a fraction of the signal lanes, say x4.
So, the vendors which were providing cut back slots were either ignorant of the spec, too cheap to pay the couple extra cents for the larger connector, or had some fundamental constrain on providing it and were willing to provide a non-conformant part (generally unlikely). I've seen these cut back slots a few times in parts of the arm ecosystem where the vendor can't be bothered to read the spec much less implement it correctly. This is also how one has the pile of problems that are frequently seen in the SBC market where the boards won't actually work with some PCIe card, USB device, whatever because the HW/SW is only implementing the convenient parts the relevant standard.