Well, the reason for that is he's one of the most versatile fighters of all time, which is a large part of what made him great.
Irregardless of his greatness, I don't think that on average Robinson did anything approaching true back foot boxing, if we're going to be literal and serious. He was the smaller man moving up 2 weight divisions past his best, how could he?
Robinson was great off the back foot imo plenty of the time despite being an essentially aggressive fighter. Light on his feet with lots of good little slides and half steps to dictate range and create punching room as well as top class peripheral movement. Obviously he was lethal and vicious as all feck when he cut loose, which was plenty of the time, but his approach was quite calculated much of the time with plenty of lateral movement interspersing the carefully timed bursts of deadly combos that he drew incoming fighters onto. If you're looking specifically for pure boxers lacking in power as the benchmark for the thread - Pep, Canto etc - then it's different. A narrower focus within fighting off the back foot, though even someone like Pep was very offensively voluminous in spite of his defensive wizardry.
Throwing together a random list with various approaches and skill sets... Lora Kalambay Rose Pep Canto Locche Ross Ortiz Monzon Hagler Jones Jr Lomachenko (for the modern crew.Wizard). B. Leonard Driscoll Midget Wolgast Sanchez Pedroza Marcel Laguna Loughran Harada in boxing mode Chucho Castillo Ali LMR Pastrano Old Bones Burley Winstone McGowan
Benitez what a master. Even though he lost to Hearns you saw his mastery in that fight. I never saw Hearns miss a body punch. He had an anticipation. Had Benitez had lesser guys to fight he would have been undefeated.
Willie Pastrano was a smooth operator, always moving, impressive considering his size. This content is protected And don't forget when Sugar Ray Robinson scored one of the best punches of all time while on the back foot.
Right now, Julio Cesar La Cruz, light heavyweight olympic champion, is a fighter that is always one the backfoot and is barely touched during fights. Laguna, Pep, Gomez, De Jesus, Toney, Benitez, Kalambay, Graham, Nunn, Ali, Pep also were masters at it.