Why can't i create more than 1 giveaway at the same time?

I think that you should level up before you're allowed to create multiple giveaways, but i might be wrong.

Every time you do giveaways that got received and all, you will get more giveaway spots.

When you are making giveaways, you can make more giveaways on one time.
So, giveaway few your games 1 by 1 and then you will be able to make more :)

for every 3 completed giveaway you get a new slot
for every deleted giveaway you lose a slot

From the FAQ:

Why is there a limit to the number of gifts I'm able to give away?

We limit the number of gifts a user can create for security purposes, and to prevent a new user from potentially posting a large number of invalid gifts to the site. When you create giveaways and your gifts are received, this number will increase, and you'll be rewarded with the ability to create giveaways with a greater number of copies, or more simultaneous giveaways.

It's to make sure that new people joining the site don't open 100 fake giveaways. The more gifts you succesfully give away (marked as recieved), the more giveaways you can open simultaneously.

oh, ok, thank you guys!

3 copies are 3 giveaways. Everyone starts with 3 slots, and gain another slot for every 3 giveaway marked as received.

If you need many slots within a short time, you are allowed to open a support ticket requesting so, which in your case might be handy so you don't have to wait for them to finish multiple times. Or you can continue at your own pace.