Why do new washers require a 2" stand pipe.

Why do new washing machines require 2" drain.

What's the downfall of using 1 1/2".

The drain is 5' away from a 3" stack vent that it will be draining into

All washing machines have always required a 2" standpipe and drain unless discharging into a laundry tub. Without proper sizing and length you will have suds overflow from the top of the standpipe. The continuous and heavy discharge from the washing machine also needs the pipe sized 2" to evacuate the water quickly so that overflow of water doesn't occur.
The plumber that recommended 1 1/2" and installs that size regularly may not be following local plumbing codes if any are enforced or exist. I know of no where that allows 1 1/2" drain as washer standpipes for the reasons mentioned.

Some older codes used to allow 1 1/2" standpipes and traps for laundry drains, but that I am aware of, all of the newer codes require 2". Even with 2" pipe, it doesn't require much of a blockage to cause the pump on a washing machine to overrun the drain and overflow. There is a good reason that modern codes require what they do, they have the experience of many plumbing contractors and plumbing inspectors put into their creation.

Even seen a customer use a full cup of detergent in an HE machine it's not pretty. Some codes require a Sud zone after the p-trap of several feet to protect the drain.