Any number of reasons really. I'll list them:
1) Flash is a bad environment to design a game in. Its not well optimized for video games. Its optimized for short looping animations, and is slowly being phased out by Adobe for HTML5 I believe.
2) Flash doesn't offer multicore support. It will run off a single processor core (1 CPU only). So if you are like me, with a Quad Core setup running at 2.0ghz per core (but 4 cores = total 8.0ghz) then you only have 2.0ghz for the game.
3) Flash doesn't allow you to use all of your RAM. I'm not sure what the hard limit is, but I doubt its more than 1 GB of RAM.
4) Konashion's game has evolved a lot over the past 2 years. The initially released game I believe did not have a body for the HER or HIM actors, as well as most of the additional effects like Dialogue, Sweat, Multiple Clothes selection, Body Slider, Auto Mode (or did it?), etc...
5) The more things that are running at a time in flash will cause it to slow down. If you are running any mods, it will slow the game down.
6) Several mods add a lot of content into the game or heavily modify variables. This causes additional strain on the game.
7) Konashion's physic engine isn't part of Flash. He built it from scratch.
8) Most people suggest using a Standalone Flash Player to play the game rather than play in Internet Explorer, or Chrome.
Take your pick of the above. Hope this helps!
