Well, it has to do with Ety's design. It's a straight through plug by default, which means it has wires that just hangs straight out and "tugs" on my ear whenever there's movement. It feels like the plug is constantly fighting to stay in, which means I'm constantly adjusting it to make sure it wouldn't fall out. Of course, I've never had it fall out, but I didn't want to take the risk of doing a test on them by "not" adjusting them and making sure they stay in.
Another, is the wire design of the Ety, which has a big connector where the two lines from the R and L plug meets. This big connector, unless properly clipped down, will ensure the extra weight will fly around and "bang" against things.. which further introduces incredibly annoying sound and weight tugging.
I'm usually wearing a t-shirt or something when I run... which means I can't find a good place to really clip them to. Clipping them to the collar is uncomfortable, arbitrary finding a spot on the shirt to clip them to is.. well.. stupid.
E2 has an over-the-ear design that helps to stabilize the wire and the overall fit. The E2 also rests inside your ear when it's properly inserted, so it's not "sticking out" like a little matchstick or something. There's a reason why all on-stage in-ear monitor has an over the ear design
The design of the plug doesn't affect how I feel about these earphones when running. The problem is mostly with the wire and fitting.
There are "over the ear" mods for the ER-4, with some stupid plastic zip-tie and such... but if you have to mod it to make it work that way, it didn't work well enough to begin with.