Can someone please clarify why only certain races are able to make a subrace on the homebrew creations? 

I would like to create a subrace of Kenku, but it doesn't give me that option. I understand I can literally copy word for word the Kenku official as a homebrew and then create a subrace from there, but I would rather just start with the official version and add to. 

Any help in the matter would be much appreciated. 

Kenku don’t have subraces.

Quote from Nrichards98 >>

Can someone please clarify why only certain races are able to make a subrace on the homebrew creations? 

I would like to create a subrace of Kenku, but it doesn't give me that option. I understand I can literally copy word for word the Kenku official as a homebrew and then create a subrace from there, but I would rather just start with the official version and add to. 

Any help in the matter would be much appreciated. 

 As SagaTympana pointed out, the official Kenny do not have any subraces. That means we cannot make any subraces for that race. If you make a homebrew version of the race and check the “has subraces” box, then you will be able to make subraces for that one. Once you add the subraces and re-save the main race, nobody will ever be able to pick your kenku race, but you and your players will be able to use your subraces.

I understand that the official version don't have subraces, that's one of the reasons I want to homebrew one. Not all of the races have variants either, but I can create a variant for any race, why doesn't the same stand true for subraces is my question. Regardless if I made a homebrew version of the Kenku and check the subraces and create my subrace from there or they make all races able to homebrew subraces without the extra step (which seems absolutely pointless), no one would be able to pick my subraces save for my players anyway.

Basically what I'm saying is that I know everything you both have said and understand it. My question, and I guess I should have directed at someone that actually makes the rules, is why the hell not? What is the point of having that extra step in there?

 Variant is designed primarily to replace raits and features of a race. The idea is that it is an alternative version of the race.

Subrace is designed to "add to" the traits and features of a race. The idea is that the base is usually not available to select because the creature doesn't exist except as a subrace. There are some exceptions but that is because of retconning between publishings.

At least generally speaking.

If you want to add a subrace, you can but for races not designed to have one you will need to create a homebrew copy of the race. This is because subraces must be built within the race.

You can make a variant of any race straight away as they can be made on their own, you just select the original race so you can, if needed, replace the features you want to replace.