As alluded to in this MSNBC clip, the strategy of at least some of the elements of the Republican party is to lay blame for the looming shutdown of the US federal government due to lack of funding provisions at the feet of Democrats. Given that Republicans currently have majorities in the Senate and the House of Representatives as well as the Presidency, this seems strange. It appears to me that for budget purposes, the "reconciliation" process could be used by Senate leadership to pass legislation providing funding, circumventing the anticipated Democratic filibuster to block such legislation. Why is this apparently not an option?
Reconciliation cannot be used here. One of the rules for the reconciliation process is that the bill cannot significantly change federal outlays or revenues. Senate Republicans want some major cuts in spending, which would violate that rule, so it would need 60 votes to pass rather than the simple majority that reconciliation requires. They only have a 51-seat majority right now, so in order to pass whatever plan they come up with, they would have to get 9 Democrats to vote for it (assuming every Republican did, which is also unlikely).
The tax bill that passed last month was able to use the reconciliation process because the majority of the tax cuts were to the business tax rate. The majority of tax revenue comes from payroll and personal income taxes, not corporations, so if you factor expected economic growth into the equation (and the additional revenues that growth would generate), the corporate tax rate cut becomes revenue-neutral which reconciliation requires. The bill had to get creative in order to fit within the reconciliation rules. This is why the personal tax cuts were so small and will expire, deductions were reduced, and measurement metrics were changed.
Even though Republicans are in the majority, it's a very slim majority, and they just don't have the votes to make large changes to the Federal budget without Democrats support.
The GOP might have the numerical majority in the 2 houses, but there are different factions within the GOP (and also within the democrats) and not all have the same views on how the government should do the budget.
Some want more spending, some want less spending.
So, some GOP members will not vote for the budget if such and such spending is included or not in the budget.
They need to have some democrats to vote with them; those democrats want different things and they need to "deal" with the GOP to have those things in the budget (DACA seems to be a big issue today); and some GOP members do not want to deal in one way or the other.
And the democrats do not need to try to get a deal, because they have the minority.