First, Matt Serra was not on a win streak. He lost to Karo, then won the fight with Lytle. Winning one fight before your title shot isn't much of a streak, and the guys he beat on TUF don't count as fights. I'm not saying that in the technical sense, but in the sense that those guys are just not UFC worthy fighters.
Sherk, on the other hand, has like a 33-2 record. If you're recruiting new LW's for your division, who better to throw into the mix? That's an freakishly impressive record, and I think he since proved that he was worthy of getting the title shot (If he hasn't been juicing the entire time). I think Sherk was a great fit for the title.