Here are some of the reasons I think Scot Brown decided not to run in the special election to fill John Kerry seat in the US Senate:

1. This would have been Browns third Senatorial campaign in two years.  Candidate energy, family strain and most importantly fundraising would have been major issues that affected the outcome.
2. It has only been two short months since Elizabeth Warren’s campaign organization drove her to victory. This machine is still hot and could quickly be firing on all cylinders again.
3. The issues and votes that plagued Brown against Warren still exist. Markey/Lynch would surely hammer him with these again. I can almost hear Markey/Lynch in the debate now…“ Mr. Brown when given the opportunity you voted against equal pay for women”.
4. If Brown is interested in the governorship as rumored, he and his staff know it is easier for a Massachusetts Republican to win the state house than a seat in the US Senate.
5. Brown couldn’t win. Considering all the factors above…Brown couldn’t win. He would have trouble raising money; the last election proved he can’t beat well-organized Democratic opposition;  And his actions in the Senate since his lose in November haven’t done much to protect him from the same criticism that help bring him down before.

Just my thoughts…