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You have to keep in mind there are more registered R’s than D’s who vote in AZ. So, any DEM candidate does not win by just getting the DEM base. They need to win the moderate R’s who they’ve been able to win over in recent times when the GOP puts in crazies like Lake, Masters, etc. If those moderate all go to Sinema instead of the DEM candidate the DEM candidate will be in trouble. 
So, even if Sinema splits off the moderate R’s this is still damaging for the DEM chances. This is the pool of voters the DEMS used to turn AZ blue in recent times. If she happens to get some of the moderate leaning DEMS on top of that (she will) this is really bad news for the DEMS.
Credit to /bobsaget824
Everyone keeps thinking she would split the ticket for dems, but an ad campaign marketing her to republicans would do gangbusters, advertise her as the one who “stood up to the dems”, advertise her over the rep choice, split their vote more