"Easier" is relative - it's all black magic to me: Ajax, all kinds of databases, and you have to think about security at every line of code.
However, to the extent that it may be easier for professional programmers, I'd say it's because web apps suck. All of them. Their functionality, speed and ease of use don't hold a candle to the current state of desktop apps. Most web apps I've seen belong in the User Interface Hall of Shame, with few exceptions - but even those exceptions can't compete with what's available on the desktop today.
The only saving grace of a web app is that it remains accessible from just about anywhere. That gain though comes at a price of potential security breaches and having to trust your data to a for-profit (or amateur-run) business. To my mind this is a poor trade-off, but that may be because I don't have to leave home to do my work.
In recent years I've seen a number of software developers (and developers of Delphi components) move their support system from nntp to the web. The results are always disappointing. It takes much longer to find anything, the search is not nearly as powerful or flexible, copying is a drag, and often you have to log in manually. By contrast, in Forte Agent I have years of experience and knowledge stored and accessible within a few clicks, and Archivarius keeps it all indexed, too.
Also in my line of work, localization vendors have been moving things like terminology databases to the web. It seems to make sense, since they can update those databases and translators always see the latest version without having to re-download or re-install anything. Yes, but then each database is located at a different website (instead of within a single desktop app), requires a different login (which they make you change every two months), and the web interfaces tend to log you out after 30 or 60 minutes of inactivity. That's like five or more repeated log-ins during a single work session. It's more than mere annoyance: it takes me longer to accomplish a task, so in effect it's like a taking pay downgrade just because web apps have become trendy.
For those who travel a lot, web apps may have improved their lives. To me though, they are but nuisance and aggravation. Yes, they are probably easy to write. It's because they're all crappy!
Edited to add: one reason web apps are loved by developers is that they can shove more adverts down my throat, too. GMail shows me ads; TheBat! does not. The choice is really simple.
And to add some more: Stack Overflow seems like an example of a really good (and very successful) web app. Information-wise, it's brilliant. But try to enter a few words in the search box and search for posts containing all of them (rather than posts with any of the words). Quick tip: you can't. I'm ever grateful to Mouser for building DC around SMF rather than phpBB, because again, the search facility in phpBB is next to useless. And it's the most popular forum engine on the planet!
They say bad money drives out the good, and the rise of web apps is a prime example.