She was in her 30s post-Charlie's Angels, mature and had proved her talent as a decent serious actress, yet, virtually all her post-Charlie's Angels work (https://en.wikipedia.org/wiki/Farrah_Fawcett#Filmography) was TV films (https://www.throughtheclutter.com/farrah-fawcett-1390.php) that were quickly forgotten and couldn't withstand the test of time.

Why didn't she branch out to cinema when she made a good name for herself in the theatre and TV, while she was still in her 30s? Was it the lack of offers or talent?

Because she decided to jump her contract after one season of Charlie's Angels, and then the producers let her go instead of doing what they should have done- holding her to her contract. Her late 1970's movies (which she should not have been allowed to do in the first place) were not particularly successful, perhaps because she was allegedly blacklisted in the US for jumping her contract, and had to mostly go overseas to get film roles. She probably could have been successful with another TV series while still in her 30s, but she didn't do another series until she was in her 40s- and by that time, all her momentum was gone.