Especially for jams, it's nice to have a low barrier to entry for people trying it out. Downloading stuff is just a hassle - much easier to just have it play in your browser. Most of the LD entries have web versions (generally a Unity WebGL build).
I had SCI0 working on the XNA framework at one point well enough that it could run fan games, and some bits and pieces of Sierra games (I was able to get the first few screens of SQ3 working:
https://mtnphil.files.wordpress.com/2012/01/sq2.jpg , and I had some complex scheme to provide menus of verbs/nouns ). I wonder how much work it would be to make a Unity "plugin" (of sorts) from the code I had, if I can find it. Seems like it would mostly just be an issue of changing how the graphics were drawn.