If my maths is correct, the Tory/DUP coalition has 323 seats.
The opposition has 320.
This means only two Tory MPs need to resign the whip to force a new Government: which presumably is exactly what they want.
So why have they not done so?
Most Tory Eurosceptics don't actually want to bring down the government. While they may be highly dissatisfied with Theresa May's leadership, they generally are even more fearful of a government led by Jeremy Corbyn and so are not willing to risk that in a general election. It has become clear that they cannot forcibly remove Theresa May as leader of the Conservative party, so the two most likely possibilities from a general election are unsatisfactory to them. While they could form a new party (or join Nigel Farage's new eurosceptic party) and try to gain power themselves, third parties are disincentivised in FPTP elections, so their odds of gaining power would be low.
Perhaps they could bring down the government and hope to command the confidence of the house without an election, but it is clear that there is no majority in the Commons for a hard brexit, so this outcome is unlikely.
They could also attempt this strategy to simply cause chaos in an attempt to force an accidental no deal, but again given the majority of MPs want a softer brexit than that, it is likely that this would be avoided at least until an extension was granted to avoid that outcome and allow time for the election.
All of this means that it would be very difficult for them to achieve their goals through this method, and clearly they have resolved that it is better to attempt to force change from within.