Why was Hodor renamed in the TV show?

As far as I know, there's no official comment. There have been two reasons for name changes from the books so far, and we won't know between them until the next book comes out.

1. To avoid confusion. This is why Asha Greyjoy was changed to Yara (to avoid confusion with Osha, who was made into a more developed character in the TV show); and also why Robert Arryn was changed to Robin Arryn (there was already a Robert and a Robb). There's already a notable Walder (Frey).

2. Because the character's backstory was sufficiently different. For some characters, their backstory and biography drifted a long way from where it was in the books and author George RR Martin (GRRM) has insisted on giving the character a new name: examples include changing Vargo Hoat to Locke and Jayne Westerling to Talisa. These characters had kept their original 'book' names in early drafts but were renamed when fundamental points about their backgrounds changed (like Locke's straightforward loyalty to the Boltons and Talisa being from outside of Westorosi politics).

The first option seems the most likely, but it's also possible the name change was to show that, like Vargo and Locke, and Jayne and Talisa, Walder's origin story might prove to be fundamentally different to that of Wylis. Perhaps Walder won't hold the door, or maybe there's some other important difference in Walder's background (there are plenty of theories about who Walder's ancestors might have been). We don't know yet.

Given the similarities between the names though, it looks more likely to be a Robert/Robin Arryn style change to avoid confusion.

Avoiding similar character names is a common practice in screenwriting. In fact, GRRM (who was a Film & TV screenwriter for much of his career) has said part of the reason he switched to writing novels was to break from constraints like this. From an interview in 2013:

We have the biggest cast in television, so we can't afford to add more characters. I have the biggest cast in literature, I think...

I wrote these books, never dreaming they would filmed or made. It was almost a reaction to my tenure in Hollywood. "I'm just going to do this as big as I want... To hell with those rules."

What rules?

Well, having so many characters, for one. Having similar names. Stuff like that... [People said], "Never have two characters [starting] with the same letter? Certainly never have two characters with the same name." But then I'm saying, "That's so unrealistic." I mean, English history is entirely composed of Henrys and Edwards...

So he was aware he was deliberately "breaking the rules" when he knowingly gave characters the same name in the books, and is still pushing the rules by having so many characters and so many similar names in the TV show.

In the absence of official clarification I am presenting which everyone is speculating about it:

In the books, Hodor's real name is actually "Walder". This is apparently a relatively common name in Westeros, but as it is also the name of the infamous Walder Frey, it seems that the writers decided to change the name to "Wylis" for the series to avoid confusion. -wikia

Also remember GoT has done this changes many time with mostly no clarification in any interviews. For example Talisa Stark, which is not even present in books

In the A Song of Ice and Fire novels, there is no character named Talisa Maegyr, though one of the triarchs of Volantis is Malaquo Maegyr. Instead, Robb weds Jeyne Westerling, daughter of Lord Gawen Westerling, an impoverished but proud bannerman to Casterly Rock. They meet when Robb captures The Crag, an ancient and decaying stronghold which serves as the seat of House Westerling.*

Even her fate in the books was different

Jeyne never becomes pregnant with Robb's heir and isn't present at the Twins during the "Red Wedding". Thus, she's alive and well afterwards, under the protection of Brynden Tully, who remains in Riverrun as Robb's Warden of the Southern Marches.*

_{Note: Talisa' example was given because of being so random not because it's similar to Hodor.}