It's not exactly true. If you are referring to Samsung only, then yes, most of their Snapdragon models are dedicated to the US, and in the US carriers are paying big money to make sure that their phones have a locked bootloader and cannot be rooted. Europe have no such problems since they don't have shady carriers like the US ones, so there is no reason to prevent a customer from unlocking the bootloader. As for the other brands, such as Motorola, they have an amazing bootloader unlocking policy which allows people even from the US to unlock the bootloader (Except for TracFone) for their Qualcomm or MTK CPUs. However, Qualcomm still has the win in Custom ROMs, because a lot of people are very familiar with the process of building device sources for Qualcomm devices. As for Mediatek, I have not seen as many fully working ROMs apart from older phones in the MTK6572 days. So for me Qualcomm wins here.