Where grass wasn't heavy, I used to throttle my mower down to reduce
noise an save gas. Now my mower has only one speed.

I think manufacturers have found that engines that can't be run slow
have fewer warranty claims. Keeping the RPMs up can mean better
lubrication and cooling.

It's more expensive to add the required throttle linkage and most people
never use that feature anyway. More often than not, it comes down to
nothing more than cost of goods to manufacture, though that isn't as
exciting as conspiracy theories.