In my role as unofficial, self-appointed late-stage Digg historian [0], it's my belief that Digg ultimately had to change as the Google SEO changes had fatally wounded its near-profitability. Further, as a VC funded company it made the inevitable (and I think best for everyone involved) decision to modernize in an attempt to be a member of the Facebook, Twitter cohort rather than experience a long-term shrinking into mediocrity.
[0]: https://lethain.com/digg-v4/