Why would they waste resources on trying to setup new Colonies? Seems like a rather expensive option.
SR reasons-
1) How many warships have the Snow Ravens been forced to convert to barracks to house their people? Even one new colony would allow they to restore one, if not more, warships to their intended purpose.
2) How are those people being fed? Most of their food is being sent up from OA colonies. A new colony would allow the SR to start back on the road to self-sufficiency.
3) Helping the OA regain some of their lost colonies would help the SR show a more benevolent side by helping both gain strength without resorting to violence. Also it would help improve their tarnished image especially after Dante.
4) After the Jihad (and their conflict with the DC), how much raw materials do the SR need to rebuild their strength? A new colony would provide them access to the needed raw material. It would also allow them to build their own factories using tech the SR are not yet ready to share with the OA.
OA reasons-
1) New colonies would help continue President Avellar's Long Road Program. New jobs and access to raw materials that could be used in manufacturing or traded to other nations.
2) There are many factions, such as some Separatists factions, in the OA that are uneasy with the SR presence. Helping the SR establish their own colonies would reduce the drain the SR have on local economies and reduce the number of orbiting warships. This in turn would make them less of a sword of Damocles presence
3) Having OA and SR working side-by-side for a common (and non-violent) goal would improve further improve relations. Most citizens of the OA are not able to see the big picture and how the SR presence has actually helped the OA.