There's a great article all about that:
How Disney Bought Lucasfilm—and Its Plans for 'Star Wars'
The gist of it is that he was getting ready to retire and knew selling to a parent company was the best bet to make sure it survived without him. So he started working on an outline for a new trilogy of films to help sweeten the pot.
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In other news:
Disney XD Soars to Record First Quarter Ratings Highs in Primetime
Disney XD Soars to Record 1Q Highs in Prime Time Among Total Viewers and Target Demographics
1Q 2015 was Disney XD's most watched 1Q ever in Primetime in Total Viewers (408,000) and across target Boys 2-11 (150,000/0.7 rating), Boys 6-11 (121,000/1.0 rating) and Boys 6-14 (154,000/0.8 rating) demographics, bolstered by the hit original animated series “Star Wars Rebels” and “Gravity Falls” and the live action original series “Lab Rats.” In the U.S., the advertiser-supported Disney XD channel launched in February 2008.
During 1Q15, “Star Wars Rebels” reached an impressive 33 million Total Viewers across Disney XD and Disney Channel, including 8.1 million Kids 6-11 and 4.4 million Boys 6-11, representing over one-third of all Boys 6-11 in the U.S.