|It ain't the rim's fault|
However; if you are a gravel rider, or "monster cross" fanatic, you know that there are basically zero choices in tires for tubeless use. You have to convert non-tubeless tires, or there are no other ways to pursue that angle. (Well....that is until WTB's Nano 40 TCS comes out)
Why is that? Well, there are two big reasons for it. One, and most importantly, the numbers of potential sales for a 35mm-45mm tubeless tire has been low, until just recently. Secondly, the technology to do a tire with this low a volume is difficult. The technical issues get bigger with lower air volume and the necessary higher pressures one needs to use to make it all work.
Mountain bike tires, with their bigger air volume, need less pressure to work. Running more than 35psi in your tubeless mtb tires? You're missing the boat then. However; with lower volume, the "spring", or air pressure, needs to be higher to keep you from smacking the rim on the tire, and to help increase the ease of rolling the tire. No big deal- just pump it up, right?
Well, actually, it is a big deal. Higher pressures mean more stress on tire beads that weren't designed for tubelessness, or even if they were, they tend to start failing when pressures rise. Just look at the traumas that the road tubeless tires have been having for an example here. So, until the missing piece of the puzzle comes- the tires- I am reticent to hit the backroads on a tubeless 40mm tire set up.