The Awesome Spitfire

Because Rainbow Dash and the other Ponyville pegasi wanted to prove they CAN do it, without anyone's help, and even with 8 of them down. Plus, it was an accident that broke the first tornado, so they didn't get to completely show what they could do, yet. Had Spitfire joined in, then they all would have felt humiliated that they couldn't do it on their own, Rainbow Dash especially. Of course, if they actually couldn't do it that second time, even when they were moving as fast as they could without any trouble, then she would have joined and/or called in some help.
This about sums up what I believe:
"With Rainbow Dash inspiring all the pegasi that they can do it, why jump in? They really wanted to prove that they can do it so she probably let them have that chance. If they really couldn't pull it off own, Spitfire would probably report it back to Cloudsdale and either join in and maybe call in some of the other Wonderbolts or get another town to do it." (From TVTropes)