DLR is franchised and is up in 2021. IIRC only renewed at the start of this year but the night tube had already been announced by then, so why night running wasn't put in as part of the agreement I don't know.
That's an easy one to answer. The CGL consortium, that built the Lewisham link, only gets paid back by 2021 and it was not affordable for TfL to "buy them out" as they did with the London City Airport / Woolwich Arsenal link. I think CGL have contractual rights to access their infrastructure to maintain it to the required standard so DLR can run trains. Trying to change the access arrangements is a step too far late in a contract term because CGL will want to be paid a fortune for changing it. There is also a big risk that reducing access time would give CGL a reason for not achieving the right asset condition / residual life in the infrastructure they will hand over to TfL in 2021. There will probably be arguments about the state of what is handed over anyway but there is no logic in taking a decision that gives a contractor another excuse for not having achieved what they were required to do.
It makes huge sense to not get in the way of CGL and to then change things for the next concession in 2021 when the entire infrastructure will be in TfL hands and interfaces are simpler. TfL can then ensure the next concessionaire plans for night operation in their bids and it is all properly assessed and managed.