I don't know... I have been taking fewer cannons overall, and haven't been taking any at 1500 points and lower. It's not just the cost of the cannons... the veteran state troops I take cost way more now too. Detachments work differently, and so does charging skirmishers, so overall my line units cost more and are less effective without adding character support. In 7th and 8th edition, I rarely used my full character allotment, now I struggle to make the characters I want fit. That impacts whether or not I can buy cannons too.
I don't know how many of these games are 1500 points versus 2000 points either. Maybe at 1500 points where Empire performs better, you definitely take a BSB, but maybe not a cannon, and that's what helps account for the win rate differential? 2 cannons plus an engineer is 300 points. That's hefty, and if I'm going for a cannon strategy I want some redundancy. It doesn't fit in 1500 points. So maybe we're seeing an artifact of point size?