Hi, please read here: SimBrief Dispatch - User Guide.
As far as SimBrief can tell, LIEO and LIRN are equally suitable. Same number of runways, 2 ILS approaches each, LIRN is slightly closer but LIEO has a longer runway which is why it is selected. There is nothing in the AIRAC data to tell SimBrief that LIEO is on an island, for example. Programming computers to make choices that humans consider “obvious” is never as easy as you might think. 
I have placed more weight on the distance when selecting alternates, so LIRN should normally be prioritized over LIEO now. LIRP has only 1 ILS and LIMC and LIML are farther away, so they are still prioritized lower.
Cheers,