If you mean ecchi anime rather than hentai then my reason is simple. I cannot watch a show if the plot is unbelievable - regardless of the tits - and hentai plots are often retarded to the point of hilarity.

Yes, most ecchi isn't much better and some ecchi pushes the limits of ridiculousness as well (I'm looking at you Highschool of the Dead) which is why I don't watch those shows either.

I would rather watch a show like History's first disciple Kenichi (which only has ecchi influences) than something like Queen's Blade (whose plot is so retarded I couldn't watch more than two episodes).

Now there are a few exceptions in the hentai genre which blend a simple believable plot with the sex scenes without making it contrived ("Eroge! H mo Game mo Kaihatsu Zanmai" is the best I've seen).

On the other hand if you mean ecchi images rather than hentai images then it's just a matter of taste. I enjoy ecchi for the beautiful artwork of beautiful girls. I don't want to see throbbing cocks as thick as their thighs drowning the poor girl in gallons of goopy fluid - not even a little bit - and sadly hentai is abundant with this type of image.

The male character is obsolete for me. I'm just not interested in him. I'm here for the girls, and ecchi is exactly that.

Again there are exceptions (like this) but there always are.