Why aren there more instruments to choose from (for example, more types of distorted guitars to choose from)?
As explained in the preceding answer, TabIt uses MIDI for playback. It is thus limited to using what instruments your sound card provides. Nearly every modern sound card conforms to the “General MIDI” specification which defines 128 standard instruments. With General MIDI, there are only 8 different guitar instruments available (numbered 24 to 31).