What are the reasons for the low returns for robo-advisors so far?

I don't know what you mean by low. I have double digit paper gains so far. If you want returns like 25%, I suggest you buy the lottery instead.

For a reasonably low-risk portfolio, AutoWealth Balanced Portfolio (40% govt bonds, 60% stocks) achieved 17.3% returns net of fees and currency impact since our inception 30 Apr 2016 to 31 Mar 2019 end of last available quarter. On an annualised basis, thats 5.9%. We have outperformed all 20 global balanced unit trusts available for sale in Singapore through Fundsupermart. You may check out our investment track record at the following url: https://www.autowealth.sg/strategy.php

If you are looking for higher returns above that, our highest risk portfolio offering is AutoWealth Long Term Growth Portfolio (20% govt bonds, 80% stocks).