Your premise is incorrect.
The 'Remain Alliance' is Liberal Democrats, Greens, and Plaid Cymru. Northern Ireland operates in a different political environment to Great Britain, and even has its own electoral system. The parties which exists in Great Britain do not operate in Northern Ireland, but they do have allies who behave as proxies with specific local agendas. The only exception is the Conservative party, who are relegated to minority status in Northern Ireland and do not have significant support. The SDLP is allied with Labour, and The Alliance Party of Northern Ireland is allied with the Liberal Democrats.
As The Alliance Party of Northern Ireland is standing in all Northern Irish constituencies, the 'Remain Alliance' is effectively contesting all seats. They furthermore are refusing to acknowledge any pacts, which they regard as anti-democratic. Though the evidence suggests that at the very least, other parties are choosing to operate as if they were in a pact.
But what about Belfast West?
As happens everywhere, Northern Ireland's constituencies vary. Some are strongholds, others are swing seats. Just browse Wikipedia's article on the region's constituencies to see relevant history.
Let's consider Belfast West. This has been a stronghold of Irish republicanism since the 1960s, first Republican Labour, then the SDLP, and then almost entirely Sinn Fein. For the last two decades Sinn Fein have won between 60 and 70% of the vote in Belfast West. It is one of the most republican communities anywhere on the island of Ireland.
So... why would anyone else bother trying there? Especially when choosing not to stand in such a place could be used to help barter a swap somewhere else, like Belfast South.
Sinn Fein have agreed to withdraw from Belfast South: implicitly to help Alliance or the SDLP as they are all anti-Brexit. So there is some sort of game playing going on locally. Curiously, the SDLP are still standing in Belfast South, and the Green Party have decided not to stand to support them, while the UUP have decided to contest the seat, which has frustrated the DUP; who are concerned about splitting the unionist vote.
The fact Alliance party is standing in Belfast West, and not withdrawing for Sinn Fein, may seem odd given the probabilities involved. But in Northern Ireland whether one is a nationalist or unionist is the penultimate political question. If Alliance party was seen to be making a pact with Sinn Fein it would alienate many of their unionist voters given Sinn Fein's absolute commitment to a United Ireland, which may be fatal.
The whole point of coalitions is regional give and take to maximise collective gains. This is why the Brexit Party have said they will not contest Conservative safe seats, and will instead go after Labour ones. There has also been an agreement between the DUP and UUP over Tyrone.
Here is a recent BBC map of which UK constituencies have majorities of less than 10%. You may notice that Belfast West is certainly not one of them. The 'Remain Alliance' has no motivation for contesting such a seat when they already have allies there. And even if they didn't, they are native to Great Britain, not Northern Ireland, and do not have the ability to stand there... as much as I'd be amused to see the Welsh nationalists of Plaid Cymru as an option when voting in Northern Ireland.