When you try to download something, you're presented with a security warning about how the software could potentially harm your computer. If you install the program despite this warning, your firewall often displays an intimidating dialog asking whether you really want to trust this application enough to let it talk to the outside world. It's a one-two punch that's driving away many would-be users of desktop software.
Maybe that's part of the problem, but I'm sure that there are other factors involved as well, such as:
- Collaboration is easier with web apps: Collaborating on a file is often done by passing it around via email messages, which requires the extra step of attaching it to a mail message, not to mention the conceptual overhead of files and filesystems. I know lots of people who when asked “Where did you save your speadsheet?”, answer with “I saved it in Excel.”
- Many web apps are free of charge to the user: For the casual user of web apps, the price tag of a desktop app may the barrier to entry; why buy something when someone's offering the same functionality for free?
- Slickness: A good number of web apps — consider the 37signals ones — are beautifully designed, whereas desktop apps are staid-looking creatures by comparison.
- No update headaches: This is really an extension of the headaches associated with installation. Just as web apps free you from having to do installation, they also free you from having to do updates.
- Easier on the IT staff: At a recent presentation by Microsoft Canada, I watched a twenty minute-long presentation just on the application and update deployment features of Vista. This sort of thing has always been one of IT's biggest headaches, and moving to a web-based app — which gets rid of a lot of installation issues — is appealing to IT.