Because the majority of MPs oppose Brexit. Period.
A Court Just Rules it Requires Parliamentary Approval:
"Highly unlikely?" Expect all of the SNP and a few Wee Parties to vote "no." Do not expect all of the Conservatives to vote "yes." Given the "Official Position" of the Cameron government was to oppose Brexit, a significant number of Conservative MPs opposed Brexit. If their constituents still oppose it . . . "all politics are local."The court ruling does not mean the end of Brexit.
The case was about the government's right to trigger the formal two-year process of leaving the EU without there being a vote in Parliament.
The government is going to appeal against the decision, but, as things stand, the ruling means MPs and lords will have to give their go-ahead before Prime Minister Theresa May starts her negotiations on the UK's exit from the European Union.
Theoretically, they could decide not to give the go-ahead - but, in practice, that is seen as highly unlikely given that a majority of people who voted in the June referendum voted for the UK to leave the EU.
The winner is May. She was not a likely PM if Cameron stayed. He heir-apparent was someone else. Okay, Someone Else pissed people off, but if the party rejected him and picked May that would be like four years from now.
Labour? If they oppose Brexit, along with a lot of other MPs, May can "regrettably" accede to "the vote of Parliament," maybe make some motions about "the will" of the people only to point to more recent polls. She can then force the EU to "eat" the reforms Cameron won before the referendum along with greater control on migration.
She can then go on to more important things free of the epic headache of negotiating multiple trade deals, immigration deals, et cetera.
If Labour supports Brexit . . . they lose a major point of their existence. Then May can blame them.
I would offer popcorn . . . but the election between HitlerTrump and StalinHillary has used it up.