The original, official statement on StarWars.com from January 28, 2013, now available only on archive.org:
Lucasfilm has decided to postpone this fall's scheduled release of
Star Wars Episodes II and III in 3D. Given the recent development that
we are moving forward with a new Star Wars trilogy, we will now focus
100 percent of our efforts on Star Wars: Episode VII in order to
ensure the best possible experience for our fans. We will post further
information about our 3D release plans at a later date.
Some additional information from Disney's 2013 annual report following the acquisition of Lucasfilm:
...Lucasfilm retained the rights to consumer products related to all of
the films and the rights related to television and electronic
distribution formats for all of the films, with the exception of the
rights for Episode 4, which are owned by a third-party studio [ed: 20th Century Fox]. All of
the films are distributed by a third-party studio in the theatrical
and home video markets. The theatrical and home video distribution
rights for these films revert back to Lucasfilm in May 2020 with the
exception of Episode 4, for which these distribution rights are
retained in perpetuity by the third-party studio.
Which suggests that the short-term value of updating the original films was limited, which is supported by Disney's statement in the annual report that they planned to focus on original content:
Our success in building a robust pipeline of original Star Wars content
for various platforms will be an integral part of our long-term
strategy to leverage the franchise across a variety of our businesses,
from theme parks to consumer products.
One can surmise, for example, that creating a 3D version of Attack of the Clones for which you have to share the revenue doesn't help you sell new toys when you already have a 2D version. In contrast, making a new movie or TV show gives you something you own 100% of the rights to as well a whole new variety of spinoff toys and theme park content, as we have seen.