A used 732 is considerably cheaper than a used A319/320.
Startups like Ryan use 732s initially for financial reasons (+its a good proven a/c - look at southwest business model).
In fact Ryan has followed the (old) Southwest business model slavishly with the pay-for-type-rating scheme!
Once you are making money, ditch the 732's and get some EFIS 737's in (733/5/8) - crews easy to train etc... just continue as before.