The more pro-European faction within prime minister Theresa May's Conservative Party is reluctant to push for a vote at this time to avoid what it sees as "political suicide"- a massively disruptive departure of Britain from the European Union without an agreement. This gives May another 2 weeks to come up with a solution.
Most observers believe Theresa May is simply using a strategy of running down the clock to the March 29 deadline. By not extending the deadline she is seen as planning to hand out to the British Parliament two starkly different options- a massively disruptive no-deal Brexit causing Britain's departure from the European without any agreement and the option of accepting the agreement the prime minister has negotiated with the EU which protects Ireland and Northern Ireland peace and open border trade flows called the Irish backstop.