Yup it's precisely because Huawei stopped providing bootloader unlock. Before that, Huawei was on a good track to be appreciated by developers. They had developers program (giving away devices), great GSI support (which means easier to make device specific ROM as well), Xiaomi's bang-per-buck. There's still a bit of development around devices of that era, but now it's pretty much dead, even though there are some ways to unlock some not-too-new devices through DC Unlocker for instance.