There are a few reasons why Jem and Scout were vulnerable to Mr. Ewell’s attack.  Unfortunately, the biggest reason is that Atticus did not take Mr. Ewell’s threat seriously.  He believed that Mr. Ewell was all bluster and would do no damage.  He also thought that he got it all out of his system when he spat at Atticus.  It wasn’t true.  He was full of deadly rage.

Atticus may not have been aware of how committed Mr. Ewell was.  The children heard some gossip from Stephanie Crawford after Tom Robinson’s death that seemed to indicate that they were in danger, but did not tell him. 

Mr. Ewell said it made one down and about two more to go. Jem told me not to be afraid, Mr. Ewell was more hot gas than anything. Jem also told me that if I breathed a word to Atticus, if in any way I let Atticus know I knew, Jem would personally never speak to me again. (Ch. 25) 

On the night of the pageant, neither Atticus nor Aunt Alexandra accompany the children.

When Halloween came, I assumed that the whole family would be present to watch me perform, but I was disappointed. Atticus said as tactfully as he could that he just didn’t think he could stand a pageant tonight, he was all in. He had been in Montgomery for a week and had come home late that afternoon. (Ch. 27) 

Jem commented that the yard was “a mighty long place for little girls to cross at night” and she should not walk alone.  It was a very dark night, described as “pitch black.”  All of these features made them very easy targets for Bob Ewell, if it had not been for Boo Radley. 

These events and circumstances lead to Bob Ewell's attack.

1. It’s very dark.
2. Atticus is too tired from having been in Montgomery to attend the pageant with them.
3. Aunt Alexandra is too tired from helping with the pageant to attend the pageant with them.
4. Atticus had Jem and Scout walk alone, not escorted by any adult.
5. Scout forgot her shoes, slowing her down.
6. Scout forgot her dress and was stuck in her ham costume, which had low visibility and slowed them down.

Fortunately for Scout and Jem, Boo Radley was watching over them and saw what Bob Ewell was up to.  He killed Bob Ewell to stop him from killing Scout and Jem.  He watched them for their entire childhood and then saved their lives.