One of the issues with large group trips or cruises is that travelers just don't know who they are traveling with.  Do travelers really want to go on a cruise ship with thousands of people they don't know?  Do they want to go on one of those big bus tours?  Some people like to travel with people they have common interests in like tours for women only or tours for adults only.  Family and friends travel is becoming even more popular these days.

Here are the advantages of small group travel with a semi-private or private group:

More Flexibility:  When putting together a private tour, the group has more say in terms of the level of quality of the the tour, the tour itinerary, and the dates of the tours.

Feeling Comfortable and Safe:  On a small group tour of 30 people or less, it is easier to screen each traveler to make sure they are compatible with the group. For instance, with a small semi-private or private group it is much easier to set a policy of "no smoking" or that everyone must be in a certain age range.  In the age of COVID-19 it is easier to set standards for a trip in terms of wearing masks or providing transportation options that promote social distancing.  

Pricing:  When booking a trip where a group charters an entire ship for instance or reserves all places on the trip just for their group, they can gain pricing advantages.