Frankie thinks that Derrick is his #1 trusted ally in the house. Why get him out when Nicole could by lying for all he knows as she has no options. Nicole IMO would definitely go after the big fish (Derrick/Cody) with no fucks given.
Derrick has numbers. Nicole can do fuck all to evict Derrick.
Frankie needs to get backdoored now, and preferably by Caleb, because you might not get another chance. And this leaves Derrick free of any fault, as King Caleb would have "thought" of this all on his own...
If Derrick spins this to Nicole, he can have her as another pocket ally, giving Derrick an "in" with every remaining houseguest, and now he is the sole source of influence left in the house.
Nicole will also help Derrick to evict Caleb, and pitting Nicole vs Christine again, only saves his ass for yet another week.
As long as Derrick keeps the heat off of Victoria & Cody in the process, he is golden.
Once it's just the 4 of them, it 3vs1 in all directions with Derrick always being part of the 3. chances are that Nicole is first to go, and then Cody, leaving D & V as the final 2.
No one in their right mind would then hand over the money to Victoria, as everyone sees her as floater.