Because securing the Chem weapons was an objective proposed by 0. The 75000 troops are part of the course of action (COA) that would make that happen. If troops are a no-go because 0 blinks due to the political pressure or the presence of the Russians, the proposed objective would be considered unachievable and rejected for that reason.
Obama did not say he was going into Syria to secure chemical weapons. He said he was going to shoot missiles into Syria to teach Assad a lesson about crossing his red line. er..he didn’t say red line, uh...the world did. /sarc
In fact, Obama has not proposed any of this other stuff people are talking about. This is how he won’t take the blame if he lied and Americans died.