Is it harder to go to Special Schools in the Army Reserves? If so, why?

"Special Schools" isn't a category other than one you've made up.
Airborne is a school tied to a unit status, an SQI.
Air Assault is a. ASI available to anyone
Pathfinder is a highly technical ASI; if you aren't familiar with the formulae of DZs and LZs, it's not for you.
Ranger School is just a leadership school. Try if you dare, not for the weak or faint of heart.

Schools and qualifications are sometimes given as an incentive. That is not always the case. Some schools are required. For the regular Army it is not that difficult to send someone to a school. I was in the National Guard and there isn't a set amount of funds for schools. In the Regular Army the Soldier is already getting paid and there is little to no expense for someone to go to a school. In the reserve components you have to bring them on orders. Just to send a Soldiers to a school for a month can cost enough to prevent that Soldier from attending AT and all of the drills that year. Unless the unit has planned for their Soldiers to go to schools then there isn't any funding to send them.

Yes, if your unit doesn't have any slots in their MTOE for that additional identifier, then they will not send you to those schools especially the USAR