How did Hamas get across so easily? Israel had "opened the Gaza border," in that area at least, under the cease-fire at the time, letting more workers cross easily into Israel than there had been for a decade. Some of these were in fact Hamas scouts, mapping out defenses and locations.
Hamas attacked the army fortifications first, shortly before hitting the civilians. Also it was a high holiday with the lowest manpower on duty.
Not all of the border has concrete barriers & Hamas used construction equipment to smash down gates & wire fencing.

It started at (iirc) 5am? Few civilians were awake at that time. Hamas came in & caused chaos, making it difficult for civilians to get to the shelters.

A lot of people living at the border are (were) very sympathetic to the Palestinian cause, they neglected their defenses & were comparatively unarmed. They were targeted for being weak, not spared for their allegiance.

Hamas "plans" dozens if not hundreds of attacks all the time. Not all are carried out.

There's always protests in Israel. It's a free country after all. People are free to demand the destruction of their own nation & capitulation to Islam. :/

Yes, I agree the response time does seem very slow, but chaos often causes that.