For me, personally, it's a matter of time. Hours in the day...
I'm just finishing up EMT-B class at a local community college. Class has been 1-5 on Mon & Weds since Jan 15th, and we'll finish up with our last day this Wed (14 May). With my two kids and a husband who's gone 2-5 days a week that's ALL the time I have in my life available to go to school. I just don't have the time for the school required to be a Paramedic *right now*. As my kids get older (they're 8 and 9 right now) I will have more time and be able to give Paramedic school the time and energy that it requires.
For me, t's not for lack of desire, I just know I don't have the necessary time that Paramedic school requires and deserves. I will in the future. In the meantime, I'll work as an EMT, gain some experience (I've never worked in medicine before, so it's ALL very new to me) and in a few years I'll have the time to move up.