Because US weakness is signal to others to act, and Hamas read (correctly) that Joe Biden wouldn't exact have a spine made of iron with this.

I’m not sure how you’ll take a rebuttal from that left-wing rag, The Wall Street Journal, but here it is anyway:

WASHINGTON—Donald Trump claims that neither the Oct. 7 Hamas attacks on Israel nor Russia’s full-scale invasion of Ukraine would have happened on his watch, and that the world was an oasis of calm and stability when he was president. The reality was more complicated.

“It would not have happened if I were in office,” Trump said in Schnecksville, Pa. “Today we are considered a joke. It’s not going to be for long, believe me.”

During Trump’s four years in the White House, there was no conflict on par with the Israel-Hamas war or Russia’s war in Ukraine. But Iran, or Iran-backed proxies, did launch attacks on U.S. personnel in the Middle East and against countries across the region while Trump was president, and he was usually hesitant to respond, wary of further attacks.

wsj.com - Stress testing trumps claims that the world was calmer on his watch

“But he was hesitant to respond”

“But he was hesitant to respond”

“But he was hesitant to respond”

Keep saying it over and over perhaps it will penetrate. “But he was hesitant to respond”. Or more succinctly, “he ran from every fight.”