In the book, as a child, Eddie's mum is depicted as extending a large amount of control over Eddie; who he hangs out with, what he does, and where he goes. She rationalizes her twisted grip over him by saying that "it's because he's fragile", but, ultimately, it's so that she won't ever lose him like she did his father (who died from cancer).
This quote, I feel, sums up their relationship nicely:
“You’re not, you’re not,” Mrs. Kaspbrak moaned. She wrung her hands... He [Eddie] began to feel his breath shorten up as he looked at her, seeing
what a state she was in, how this latest escapade of his had hurt her. He wanted to tell her to
take it easy or she’d have a heart attack, but he couldn’t. His throat was too dry. “You’re not
all right, you’ve had a serious accident, a very serious accident, but you will be all right, I
promise you that, Eddie, you will be all right, even if we need to bring in every specialist in the
book...” It, Eddie's Bad Break, 4
27 years later in the book, as an adult, it is shown that Eddie still hasn't moved on; he ended up marrying a woman (Myra Kaspbrak) of acute likeness to his mother (still overly protective of him/similar appearance).
This similarity is summarised through these two quotes:
He [Eddie] looked from Mother to Myra and back again to Mother.
They could have been sisters. The resemblance was that close. It, Six Phone Calls, 4
But in the end he [Eddie] had married Myra anyway. In the end the old ways and the old habits had
simply been too strong... Myra had
condemned him with solicitude, had nailed him with concern, had chained him with sweetness.
Myra, like his mother, had reached the final, fatal insight into his character: Eddie was all the
more delicate because he sometimes suspected he was not delicate at all...
It, Six Phone Calls, 4
I feel that the mini-series seeked to further emphasise (without changing the underlying plot) this lack of progression between young and old Eddie Kaspbrak by showing him as still living with his mother. Also, the fact that he's still a virgin reinforces the fact that Eddie is still trapped within her "sphere of influence".