TF1 mentioned it... twist rate for factory .270 Win rifles is slow, which limits high BC bullets due to their length. There are some .277 bullets with high-ish BC values, but be careful as factory barrels may not fully stabilize them.
For example, JBM lists the 145 ELD-X at 1.367". Using that value in the Berger twist calculator, along with 2970 fps (advertised by Horny for Precision Hunter ammo), 1-in-10 twist, and ambient temp of 25 degrees F, you'd need to be hunting at 6000+ feet of elevation for full stability.
Whether full stability matters or not, is up to the individual but it can affect the actual BC. Of course, you can correct for it based on your environment but varying BC is a pain for some and a non-issue for others depending on where they hunt.
