It has to do with your country! At each country there are laws, different or unique from other countries. This means that perhaps your country's laws don't allow that kind of transaction to occur.
Also to buy with SMS must a telephone company or other provider, to have an agreement with imperia online company about the pays. Perhaps IO wasn't able to reach such agreement! I am sure that IO wants to be easy for anyone to buy diamonds...this way will not lose any profits!
You can always use the other, international, ways!