In Harry's second year, when Muggleborns were attacked, why didn't Dumbledore set up security measure in Hogwarts right after Colin Creevey's attack to keep the rest of the Muggleborns safe? He could've asked the staff to implement rules like 'students are to be accompanied to each class by a teacher, quidditch is to be temporarily postponed', and such, right after Colin Creevy was petrified.
As far as I know this can only be answered through speculation. Best answer I can see is he didn't want to cause a panic, and he had to be sure that it was indeed the Chamber, rather than some form of imposter or thug attacking Muggleborns under that name. Remember, he suspected Voldemort opened the Chamber originally...
"But, Albus ... surely ... who?"
"The question is not who," said Dumbledore, his eyes on Colin. "The question is, how . . . ."
but he knew that it couldn't be him, since...
"What interests me most," said Dumbledore gently, "is how Lord Voldemort managed to enchant Ginny, when my sources tell me he is currently in hiding in the forests of Albania."
At this point he likely judged it in the best interest of the school if he personally investigated, rather than causing a school-wide panic by ordering a lockdown - especially since the lockdown might not help, as they don't know how students are being attacked.
He knows well the power of mistrust and fear, as we see both in the next book and in the Second War. Even without the lockdown the students start getting jumpy and blaming each other! His apparent inaction was deliberate, as McGonagall states later:
"Professor Dumbledore's instructions were to keep the school running as normally as possible"
From a political standpoint this remains true as well - if Hogwarts starts implementing these rules it makes the attacks look extremely serious, and outside agencies start getting involved. Look how quickly Malfoy was able to turn it to his advantage to get Dumbledore sacked (yes, with the help of some blackmail and threats, but still).
On top of this, since there is a possibility Voldemort is involved you can see why he'd be be reluctant to start attracting attention from the Ministry - they'd either not believe him and scapegoat someone, or panic and close the school, both allowing Voldemort to win AND stopping Dumbledore from investigating this mysterious power he is wielding (which of course later proves to be crucial to the series).
Of course, after Hermione is attacked he obviously judged it serious enough to implement just those rules that you've mentioned.
"All students will return to their House common rooms by six o'clock in the evening. No student is to leave the dormitories after that time. You will be escorted to each lesson by a teacher. No student is to use the bathroom unaccompanied by a teacher. All further Quidditch training and matches are to be postponed. There will be no more evening activities."