Multi city question
 

I live in Nashville and B6 only has a few routes out of BNA. Their route network is pretty good for the Caribbean though out of FLL. I started to book BNA-FLL-PLS using the multi city tool page but when i got to one point the site told me I needed to book these as separate tickets. What is the point of the multi city booking page then? Sorry if this dumb question I have only flown B6 once and really like it over SWA which we fly a lot. I am not sure what to do because I thought I could do it this way. Would love some advice. We would certainly fly B6 more if they would just add more routes out of BNA :)