Because she was weak and she did not have the brains to stand up for herself. She probably had low self esteem and was in awe of her husband's appearance. It was reported that he was a strong willed person who also influenced his younger brother also. He learned all of the evilness when he returned to Russia.
I believe that she is a radical islamist. Same beliefs as her husband with the same hate for America. She distanced herself from her parents and friends so she could be one with him. If this where not true she would be cooperating with the investigation. She gave up her dreams to live his jihadist ideals. She worked to support his activities with full knowledge of what he was planning. The only important goal for her was for husband to succeed. It matter not that her daughter was in danger in their apartment while the bombs where made.
If she was brainwashed, she would ditch this religion now that her husband is already murdered. She will abandon this religion if the FBI can intelligently prove that Tamerlan was the real bomber. All evidence presented by hundreds of pictures circulating on the internet point tot Tamerlan and Dzokhar being patsies.
Russell fell in love with a man, just as any other person falls in love. She wanted to share everything in life with him so she converted to Muslim. I believe he made her life very hard and he treated her more like a slave than a wife. She worked 70 hour weeks while he stayed at home. It seems as though this was the life she wanted, but who knows what was going on behind closed doors?
I don't believe Katherine Russell Tsarnaev was brainwashed by her husband. I don't believe brainwashing is possible. I think each person is entitled to his or her own opinion and can't be told what to do or believe by someone else unless they believe in the same as the other party.
"Brainwashed" is a very serious term, and if Ms. Tsarnaev is living on her own now, and has held a job outside the house, then chances are that she has not been brainwashed by her husband. Unfortunately, women may become subordinate to imposing men, but we don't even know that that was the case here.
I do not believe that Katherine Russell Tsarnaev was actually brainwashed by her husband. I believe that she probably fell in love with him, and was willing to do whatever she could to make him happy. Don't we all do that for the person that we love? We can't call that brainwashing, and she should not be blamed for who she married.