First of all your DM does not have the ability to allow your character access to these sheres. If that were true there would never have been a need to write up the gods.
A DM must stay within the officila rules of the shared world and the core books unless they have been psoted as different or as '
house rules' for thardferr.
As for Kreaon, his first description hints greatly at why he hasn't the spheres you mentoin.
Kreaon, commonly known as the Creator, is thought to be the first god of Kyranath. He was the one who calmed the chaos at the dawn of time. He is said to have the greatest power of all the gods, but unlike them he rarely involves himself in the workings of mankind. He is seen to be withdrawn and reclusive, not emotionally involved in the present manifestation of the world. But never the less his hand is in everything and his beneficence shines over all who walk his creation. It was his hand that set the stars in the sky, the moons to orbit the world and the great sun to give life to Kyranath. The sun that brings warmth and light, and allows the plants to grow is said to be his eye. Though emotionally disconnected his benevolence and the laws he put into place at the beginning of time are the greatest gift of good ever granted.
Kreaon is the reclusive far off sun and stars, the hand that set tings into play but then stepped back to let others work on the details. He doe not deal with things like plants and animals which are transient but on the greater workings of the cosmos and the infinite.
I believe the dogma mentoined after this continues in this leaning of seeing the bigger picture and not dealing with the things growing or running on the planet but on the physical laws that govern the workings of the universe instead.
The picking of the spheres was not arbitrary and Kreaon has more spells and spheres than most every other god. He has more major spheres than the others only one minor, combat. Aerie, for example has 8 major and four minor while Kreaon has 12 major and one minor.
one of the flaws with our god write ups is the large number of gods that were linked to Thardferr. this forced the gods com to have to separate things more than most other pantheons may have needed to do. Certain gods are more aligned with nature and plants and animals and had to have these spheres. this made it so that some gods that really didnt' need them or were written to not be associated with them directly to not have them.
Comprimises were needed to be made and things , I believe and most others have agreed, have turned out to be very well balanced.
In our discussion on Open RPG I asked that you post your thoughts that certain spells should be included in other spheres. I agree that this may be a good thing since some of those lists in the PHB seem either to be in error or not very well balanced. But I have to disagree that Kreaon is flawed in not allowing these spheres or in having less spells than others or that things are arbitrary. A lot of people smepnt alot of time working on these things and nothing was left to being arbitrary. If anything we over thought things not under thought them.
If you would list the spells you believe should perhaps be also included in other spheres that would be a thing we could perhaps deal with much more esily than trying to rewrite the gods again.