The most commonly used and easy to read fonts are currently included.
More fonts may be available in the future. One obstactle is that most fonts are actually owned by somebody and cannot be used in a software program without paying royalties to the copyright owner.
Because of that, we want to be sure that any font we are paying royalties on is needed and actually gets used by a substantial number of people.
Another issue is that for a font to be seen by visitors to a site, the font has to installed on the computer that the visitor is using. If it's a non-standard or irregular font then the text will just appear to them as Times New Roman.