What site are you using? Anyway, stick with trenitalia.com Resellers tend to be "creative" when they can't access the official timetable.
You see "strange" or limited routes because you have entered a day of travel past June 12, the day of the summer timetable change. As you can see if you use a date before on trenitalia.com , the best "path" is a direct local train from Bergamo to Milano Centrale and a direct IC train from Milan to Monterosso.
According to the winter schedules, there are 7 daily IC trains from Milan to Monterosso. In summer the level of service can only improve, but I doubt they are adding more trains to the seaside on Mondays.
The journey takes 3 hours, plus the time from Bergamo To Milan (around 1 hour, I guess).
ps To be fair with the Bricklayers city, Bergamo station is connected with the u/c Milan-Verona high speed line and some Freccias stop there each day. Pota!