Because they're cheaper. I lived in counties that formerly used mechanical voting machines. They were replaced by electronic ones when the cost of maintaining them became higher than the replacement cost. If security was considered at all, I speculate that it was not the highest priority.

I agree but only if paper voting is abolished, otherwise you'll have both systems in place. Or is the paper only used when there are concerns about the results?

Exactly. They provide nothing over the security of a paper ballot.