If I were to guess, it's because most triathletes use a road bike purely for training, so the aero gain isn't worth much to them. Also, from what I've heard, the new tarmac is as aero as the first generation venge, so I can't imagine there being that big of a difference in drag between the latest venge and the latest tarmac.

yes, you're probably right.
theory is its about 10W difference, partially due to the wheels and bars which can of course be used on the tarmac narrowing the gap to about 7.5W.
the weight difference is negligible so it comes down to ride "feel" and handling. i expect the tarmac will be better in those respects but the question is how much and there seems to be a variety of opinions ranging from unnoticeable to moderate

In all honesty, I think the difference is higher than 10W (even with the same handlebars and wheels and position on both bikes). I really loved how it flew on relatively flat terrain. I don't think the weight difference is negligible (it will be close to a pound different - hell my sl6 is almost a pound lighter than my sl5). Unless you are a bigger rider, I really don't think you will like how the Venge climbs. The Tarmac is a much livelier bike when climbing. I think the more aero shaped tubes and extra stiffness of the bike is what does it. I just wish someone had talked me out of the Venge. I bought in to all the marketing crap about the aero benefits outweighing it's extra weight when climbing and all. Turns out there is more to climbing than aeroness and weight of the bike.

very useful thanks, it is exactly this less measurable feeling i am trying to get to the bottom of
the weight difference depends on the size but i understand the venge is lighter than the SL5