It is either not available as it is already booked around Christmas time and/or it is cheaper to hold it in NBH.
The latter is most likely the main reason in my opinion, as the BBC has spent a lot of money on using Studio D at Elstree since 2015 - The 2015 Election, The 2016 Referendum and The 2017 Election all came from the studio in the space of two years.
Studio B at Broadcasting House proved effective with the local and European elections earlier this year, so they know it can be used well for the general election.
The 2019 general election will be the first time BBC One has not used a huge studio set up for a general election night coverage. The big mega sets have nearly always been used, first in TC1 at Television Centre until the 2010 election, thereafter at BBC Elstree.
I think 1983 and 1987 were the two general elections with a more basic, smaller set up, however since 1970 most general election night studios at the BBC have been massive. This will be a change for 2019.