I don't know that we will ever be able to discern the "whys". There are several factors...actually many that may contribute to these crowds.
1. Pent up demand by the infrequent visitor. For example, a family that goes to WDW every two or three years recovering from a "reversal of fortune" in 2008 has finally been able to save enough or justify going into debt after a number of "staycations".
2. Gas prices are averaging in a range not seen since 2009. Families within driving distance may have made their decision based on the relative cost of this year vs the last several years.
3. New stuff to see in Orlando (Diagon Alley, FLE) since their last visit a couple of years ago.
4. Increased attendance by foreign tourists due to favorable package pricing and their current ability to pay (subject to the world economy).
5. The familiarity factor. In other words, guests who previously went to Disney resorts and had a predictably good time not wanting to chance the beach, mountains, national park would appeal to the varied interests of their family.
6. Changes in school year scheduling. The old "Memorial day to Labor Day" summer holiday really doesn't exist anymore. Also, on some threads on some message boards I am seeing more of a reluctance on the part of school districts to permit "unexcused absences".
7. Renewed interest in "loosening" credit criteria. After a hiatus on issuing additional credit, with new programs such as HARP, banks are loosening the purse strings once again and falling into the same old "sign up for this credit card for a low introductory rate". HARP loans are locking people into new 30 year lower interest mortgages to save 200-300 dollars a month etc.
8. The "Bucket List" mentality. I personally know of many people who, this summer are taking "Bucket List Vacations" (although not to WDW). There is some pervasive attitude of "you only live once" or "now or never" that seems to be infecting these people. Once upon a time, delayed gratification was the pervasive attitude. Somehow, the "life is short so live it now" is where all my friends and family are at. I feel silly for meeting with one of my CFPs (Certified Financial Planner) today to discuss long term goals in my "portfolio". My friends don't have one. The attitude that "nobody is going to let me go hungry or die" seems to be infiltrating even the middle class.
9. The horrible winter(s). Many in the Northeast and Midwest have suffered through two successive winters of crazy snow and cold to the point we are wondering why we even live here. If summer is when we can go to Florida, we'll go in summer. Not to mention, 90 and humid isn't exactly unheard of "up north" in the summer. We can deal. We already deal with 140+ temperature differences in a given year.
10. Disney Marketing and "synergy". Okay, don't shoot me. This is me tipping my hat to dragging out the Frozen phenomenon and the success of "Once Upon A Time". Cross marketing, IP placement and earworms are effective or companies wouldn't use them.
I'm sure there are more. After thinking about it for an hour, this is what I came up with.
And I thank you (and everyone else) for the thoughts.