Reasons -

Cleaner (little or no fuel "goo" when using gasoline)
Easier to see (eyes getting old)
More realistic flight characteristics (slower, thumbs getting old too)
Engines less finicky and more realistic sound (gasoline)
Construction techniques (I like to cover w/ fabric/paint over built-up frames)
They LOOK GOOD
More for the money - doesn't really cost much more than smaller planes/engines
Easier to "detail" scale models
If "scratch" building, can use a lot of "off the shelf" components like woods, fabrics, paints, fasteners, etc.
Less need for "micro tools", shop tools are fine
Fuel is cheap and available everywhere
Engine conversions are cheap and fun to do (weedies)
I have about 50 1/4 scale and bigger kits in storage

Drawbacks -
Transport (not a problem for me as I always have a large vehicle)
Storage - need a LOT of space, might not be as big a problem for those with a basement, but I live in Florida (NO basement)
Building space - need bigger building board
Building times can be a little longer simply because there is more to do