My 4-5 year old el-cheapo "Deltajet" (cheapest I could find at the time, for hosing mud off the bicycles) is a 1200W induction motor.
In the case of lawnmowers, I suspect the use of universal motors is to make mowers into consumables, which people will thus tend to change every few years. Just about every characteristic of a universal motor is inappropriate for a lawn mower. You can find mowers with induction motors, but they tend to be in the better quality less well known makes, which you have to buy from specialists rather than DIY sheds.
You can also get a more powerful universal motor in a smaller physical size, but generally they are significantly less efficient. E.g. you need around 30-50% more powerful universal motor to get same cutting power out of a lawn mower as you do from an induction motor, and the universal motor will run too hot to touch even with forced air cooling, whilst the induction motor only runs luke warm. I don't know if this is an inherent design issue, or if it just reflects that the induction motors are found in better specified/designed products. Also, universal motors generally run much faster, indeed much too fast for many applications where they need to be geared down (e.g. mowers, drills, etc).
I don't see why they should be technically. It may be a question of supply and demand, what the market will bear, etc.