So what made mint successful with their frontend then? Perhaps Yodlee hasn't tried hard enough for the frontend.
I'm going to guess that Yodlee made the switch around the time of the B2C collapse in Silicon Valley. At the time many companies attempted to redefine themselves as B2B companies believing that to be safer (essentially consumers expected web sites to be free, whereas companies would pay for services).
Also a lot to do with inflow of online ad dollars in finance vertical..look at bankrate.com earnings/stock history.. btw, speaking of minimal functionality required and just doing what is necessary for successful business - look at bankaholic.com which was acquired by bankrate (On personal level Johns Wu might have made near about equal if not more money as Mint.com founders -- given it was profitable company). Also, that is even better story compared to this -- Bankaholic gets millions by paying $0 for WordPress.
Yodlee's frontend was ugly, clunky, and unfocused. Probably still is. UI was not their strength.
Yodlee's current frontend is nothing flashy, but it works extremely well and is very straightforward and stable. Of all the online services I use, Yodlee is among the very best. It saves me many hours of time every week and has dramatically increased my control over my personal finance information. (BTW, it is easy to export your Yodlee data in CSV format for local backup and further use in Excel.)
So, it was a distinct choice on their part to go for the back end and abandon the Mint-like interface.
And they weren't alone in trying to be Mint in 1999/2000. Anyone else remember ezLogin? IIRC they had an exit of around $120m in 2000.