"Why do you keep fighting?" the radio man asked Muhammad Ali.

A slightly incredulous expression crossed the countenance of the heavyweight champion of the world. "You know what they're paying me for one night - $2.75 million. This is not Joe Frazier or Ken Norton or Jimmy Young," he said, forgetting for one uncharacteristic instant that he was supposed to be pumping his bout. "I'm getting $2.75 million for a tuneup, a warmup, against a nobody."

Ringling Brothers folded their tents and marched the elephants away from the Beltway Saturday night, but another three-ring circus came to town yesterday. Ali arrived to begin training for his May 16 title defense at Capital Centre against the aforementioned nobody, Aliredo Evangelista, but who's to argue that the champ alone is not the greatest show on earth?

Ali, who pointed somewhat forlornly to a roll of flab around his midsection and said he wants to get his weight down from 228 to 220 in the next 10 days, begins his daily 1 p.m. workouts in the ballroom of the Sheraton Inn-Northeast Washington today. Admission is $4 for adults, $1.50 for children under 12.

Alfredo Escalera, who will defend his World Boxing Council junior lightweight crown on the three-bout card that also includes lightweight champ Roberto Duran in a nontitle 10-rounder against Javier Muniz, will train fron noon to 1. Evangelista is scheduled to follow Ali at 2:30.

Ali was relatively subdued as he sat in the hotel lobby yesterday, chatting with reporters and passersby; signing autographs, posing for snapshots, flirting with every attractive woman, even though his third wife, ex-model Veronica Porche, sat a few yards away.

They arrived in the morning from Ali's camp at Deer Lake, Pa. Gene Kilroy, his chief-of-entourage, drove up at midafternoon in a Lincoln Continental with a U-Haul trailer bearing gear. Kilroy was accompanied by cornerman Drew (Bundini) Brown and Jimmy Ellis, who will join Jody ballard and Mack Smith as Ali's sparring partners here.

The visit did not begin pleasantly for Ali. He had to cross a picket line of six hotel employees who want Local 25 of the Hotel and Restaurant Employees to represent workers at the nonunion hotel.

The champ had misgivings. He talked to the pickets, asking them what they were seeking. "Sick leave, benefits, things like that," he was told.

"Do they have that stuff on most jobs?" Ali asked.

The pickets nodded, and Ali went back inside to talk with hotel manager Bob O'Neill.

"You've got to understand that he asks those questions because he ain't never had any job," explained one of his bodyguards, Lloyd Wells, who wears a Super Bowl ring from his decade as a scout with the Kansas City Chiefs.