Ostensibly, because they are unwilling to accept any peace agreement that leaves territory they consider to be theirs under what they consider to be occupation.
Several sources imply this, but this source is the most explicit as to potential motivation:
The SAF refused to attend the meeting as long as the RSF occupies Sudanese cities and villages, dashing hopes for a cease-fire deal. Sudanese military leader Abdel Fattah al-Burhan reiterated this position on Wednesday, hours before the scheduled talks.
“Military operations will not cease until the last militia withdraws from the cities and villages they have violated and colonized,” Burhan said in a statement.
Taking SAF's leader's statement at face value, they have established their precondition to commit to a ceasefire negotiation. Reading between the lines, use of the verb 'colonized' implies Burhan reads the current military situation in a particular light that invokes concepts of justice and sanctity of territory - concepts that are difficult to abandon for the sake of peace.
I say 'ostensibly' because negotiations during armed conflict is never cut-and-dry.
It is entirely possible SAF and RSF are currently negotiating. Leaders, especially those whose positions depend on support from one or more factions, often say things publicly that run directly counter to their actions or statements in private. Negotiations for peace often happen in total secrecy, while both sides zealously pursue military objectives against each other.
Admitting to participating in negotiations can be a morale-killer. Consider the case of a small nation that has been invaded by a foreign aggressor, lost much territory, and is very close to total collapse of its military. Informing the public that you are negotiating an end to hostilities - one certain to require that your country cede territory thousands have died to retain - could be the very thing that causes your military to crumble and you lose everything as a consequence.
Negotiation between international actors, and especially belligerents, is often a delicate thing. Often the talks are fruitless, but in the spirit of 'you never know until you ask,' talks still happen. Nobody wants to be in a shooting war.
Burhan's statements, in this more nuanced view, could simply be a signal to RSF and the sponsors of the talks that the political situation within SAF will not permit open negotiation, and so if RSF and the sponsors want that they're going to have to give Burhan something he can point to that allows him to attend without losing face. Burhan knows the RSF and rest of world are watching him make those statements. Thus, public statements like that are a form of diplomatic communication.