Partially because it was what I used when I was first introduced to Scheme. It has the basic Lisp-editing features (paren-matching, indentation); it colors text (e.g. comments are orange, literals are green, normal code is blue); it has a nice GUI for customizing much of the above (for indenting, you can give it specific words or regular expressions to determine whether to indent something 'define-style, 'lambda-style, or 'begin-style; you can also pick your own colors instead of the above, though I don't do that). I also like that cmd-I reindents everything.
Drawbacks: Indentation can't be customized as much as I'd like[1], at least as far as I know. Tendency to freeze up for several seconds (maybe because I tend to leave several files open for days at a time, but I'm not changing that).
I've experimented with Emacs too, and they are pretty comparable. I use DrScheme these days mainly out of habit, but one obstacle in Emacs is that I don't know how to make it do Arc-specific indentation, whereas I can do that pretty well in DrScheme.
[1] For example:
(with (this (blah)
that (thing)) ;this line is too far right
(do-stuff) ;this line isn't, because I told it
(do-more)) ; to indent "with" lambda-style
