This is normal. Zaps are essentially just another form of voluntary interaction, such as likes or boosts or replies, so they would tend to come from the same minority (sub 10%) that performs all other interactions.

It's a question of active/passive involvement.

This is not the problem. The problem is that the 5% is too small of an absolute number, to add up to significant income over zaps. If we had 1 million daily users, 5% would be 50k zappers. With 2 million, it'd be 100k.

So, the goal should be to expand the number of users in toto, perhaps by promoting more diversity of content, raising software quality, and developing apps that have wider appeal than just-another-microblogging client.