Lots of reasons, really. When flying, one has to take into consideration the baggage restrictions by the airlines, the space limitations in both the vehicle that will get you to and from the airport, access to laundry, and whether or not you'll need to buy some different clothes than you already have for the trip.
Airlines have different baggage restrictions based on where you're going and what class of ticket you're using, so be sure to check with your airline for size and weight limits as well as fees. Delta's standard baggage allowance permits us to each take one bag no more than 62 inches in size and weighing no more than 50 lbs for free because we are traveling from the US to Europe. The problem for us is that we cannot fit four bags that size in our car along with the four of us. Plus we really don't need to haul 200 lbs worth of items to Europe! Plus we don't have that many outfits!
Another consideration is whether you'll have the ability to do laundry while traveling. Call us crazy, but we don't mind doing a load of laundry every few days, and many times we come home from vacation with only the last day's clothes needing to be washed. When figuring out how many sets of clothes to pack, we count the number of days from the time we leave home until the first time we can do laundry. And then we check the rest of our itinerary to see when we could do our laundry during the rest of the trip. The highest number of days between potential laundry stops determines how many sets of clothes we pack.
In the past it seems like our boys have hit a growth spurt just as we are ready to pack for a trip. We have everyone try on all of the clothes they are planning to take to be sure that they fit properly, they are not in need of repair, and that they can make acceptable outfits from them (no blue and orange together, please). We don't pack formalwear for dinners, but we do bring suitable attire for the onboard dining rooms, and if there will be clothing restrictions for any of our tours, we make sure to have that as well - such as knees and shoulders covered for church building tours.
We do know people who bring their clothes that they are ready to be rid of and wear those throughout their vacation and toss them along the way, leaving plenty of space in their luggage for souvenirs. And others who go to the thrift store and buy $1 t-shirts and shorts so that if they do need to leave clothing behind to bring home their souvenirs, it's no big deal.
For this trip, we're figuring we'll need enough clean clothes to get us from home to the ship, so with two days travel, two days in Barcelona, and then the day we board, we'll need at least five sets of clothing. But we never travel without a spare - you never know when your Sprite might explode or another passenger might spill something on you.