I guess I used to fall into this category, and can speak more to this effect.

I think the reason for "Why Dropbox?" can be broken down into several stages:

Why Internships?

As a college student in Computer Science, internships are a large part of establishing a firm footing in the industry. It brands us as employable, and in Computer Science, unlike many other streams today, internships are almost a necessity - not just your typical Junior year one, but sometimes all the way from Freshman year. Basically, in a hyper-competitive industry, internships set you apart, and they are a big deal.

Why brand names?

Most college CS majors believe that it is the brand name of your internship that matters much more than the actual content of your internship. People would rather do a trashy internship at Google than a solve a deep hard problem at a startup. Especially for internships, I agree with this logic. During my full-time or even internship interview phase, it wasn't hard to see how easily you jump off the page with a big company name on your resume. When applying to tech companies who receive a huge number of applications, things like college name and ex-internship company brand names are often used as filters to identify top-notch applicants. This stems from the bias towards false negatives than false positives as outlined very well in Nathaniel Roman's answer to I was told after 10 minutes in an internship interview that I'm "just not cut-out for Facebook." I'm in the top 5% of a computer engineering program at a large university, and I have a year of experience. What can I take away from this feedback?.

The Community.

Here's why. The CS community is large and very well connected through ex-internships, high-school friends, and online forums like Quora. It spreads though many colleges, most prominently MIT, Stanford, Carnegie-Mellon, GTech, UIUC, Penn, UCBerkeley, and UMich. There are smaller delegations from places like UT Austin, UWash, Caltech, Princeton, Harvard and Cornell. Pardon me if I've missed out a few. My point is, the size of this discrete set of major CS colleges is about 10-20 tops, and inter-college connections happen all the time. Word spreads quickly.

The Hierarchy.

I've established that brand names are important. Now you ask, why doesn't everyone just stay happy with Google or Facebook or Microsoft or Amazon?Because news spreads so quickly in the Computer Science community, we hear about a lot of things - from new startups to trending topics in CS. One thing we hear about a lot is 'who is interning where?' Naturally, we develop a company hierarchy in our own minds. If person X chooses A over B, we see A as a little greater B. If 9/10 people who had the choose A over B, we see A as clearly greater than B. If X is a computer science legend and chose company Y and stuck to it, we begin to take note of company Y.
In much this way, we implicitly establish a hierarchy of company preferences. This hierarchy varies from college to college, but it is surprisingly easy for a more-experienced person to impose his hierarchy onto others. An innocuous statement like "I'm interviewing for Quora." when the person already has a job at, say, Microsoft, would leave the other person re-evaluating his hierarchy accordingly.

Why Dropbox?

So, to answer your question, in my experience, merely because of this hierarchy that's formed in my head because of so many of my intelligent friends choosing Dropbox or complaining about the difficulty of their interview, Dropbox has risen to an elite status in my mind. I can speak for quite a few people when I say this is usually the reason most people are "crazy about a Dropbox internship".