Robo-advisers are subject to the same regulation as traditional asset managers - a regulatory burden which is only likely to increase over time. The legal fees associated with regulation are not cheap.
Compounding the problem, a competitive robo-adviser needs to offer lower fees than a traditional asset manager. Probably they need to be in the 0.25-0.5% range, which means that for each $1m under management they are picking up $5,000 of revenue. Out of that they need to pay salaries, infrastructure costs, legal costs, rent, taxes etc. Let's be conservative and say that these costs are $2.5m per year. That means that a robo-advisor needs about $0.5bn under management before it begins to turn a profit.
I don't think it's really suitable as a side project.