The impression I got (I just finished a re-read of this classic) is that her over-protectiveness was at least partly the result of her husband's death. She was clinging desperately to Eddie because he was all she had left.
It's important to note that Eddie eventually escaped from this smothering protectiveness, but it's interesting that he married a woman similar to his mom.
I have seen people do this many times in real life, and the explanation that I have read (and which makes perfect sense to me) is that the person is basically still trying to fix that dysfunctional relationship with the one parent by marrying someone similar and making that relationship work. Not consciously, of course, but subconsciously.