One of the reasons is it's hard to find good content.
There are very few topics on Steemit that has big community. These topics are mostly about Steemit and cryptocurrencies. Steemit's scale is small and it's reasonable to assume people will stick with reddit etc. where they can engage with other people, where their questions can be answered well and fast.
Another reason is Steemit UI needs to improve a lot.
I think a problem with topics will be solved when more people come. But UI... I agree with you but I don't know how could I help with that since I am no developer...
I'm pretty sure you can be helpful if you have good ideas. You can recommend new features or designs to the development team at https://github.com/steemit/condenser/issues
Thank you! I will surely check this and see how I can help. :)