Is there any out-of-universe reason why Delos in the new series had no other theme parks apart from Westworld, as there were in the 1973 movie?
Out of Universe...because the producers want it that way.,.but there might be other worlds that we haven't seen...or these might appear in the future.
Independent.co.uk [October 2016]
The Wild West-themed park that Westworld centres on offers plenty for us to try and get our heads around, but it sounds like future seasons will see Guests explore new environments.
In the original 1973 film, parent company Delos operates three areas: Western World, Roman World and Medieval World.
Asked this week whether we’ll ever see the other two, co-showrunner Jonathan Nolan said: I've been coming to New York Comic Con for seven years and I've never said a single thing of substance. They keep inviting me back but I've never answered a single question, not really. Does that answer?"
He was obviously keen not to divulge any spoilers, but later followed up: "You said Roman World and Medieval World, right?" When the fan nodded, he said: “No.”
We can fairly safe assume from this that while those two won’t feature, he and co-creator Lisa Joy are cooking up other worlds
In an interview with Inverse, the “Westworld” production designer hinted at other worlds showing up in season two or beyond. “For the first season, we only explore the West World,” Grobler told Inverse.
“There’s talk about in the future seasons, if there’s more, that there will be a different world. But we’re not sure what it will be yet.”
Also note that Westworld was planned/pitched as multi-series from the outset
We didn’t want to have a story that repeated itself [each year]. We didn’t want the Fantasy Island version of this [where new guests arrive at the park every season]. We wanted a big story. We wanted the story of the origin of a new species and how that would play out in its complexity.
There are a few reasons given implicitly.
First - The hosts creator, Ford, doesn't want to give up his source code (as we can see by the troubles they go trying to get the source code out of the park). So this limits the area of action to where Ford is.
Second - The financing on the hosts is not because of profit (the money the guests pay). Quite in fact, the exorbitant price a Guest pays, does not turn into profit. The administration has an ulterior motive for the experiment, which is why they keep financing it. The interest is based on the source-code (again, as evidenced by their attempts to retrieve the code), not on the era of the simulation. So there is little reason to expand to new parks and eras, when they already can get everything they want from just one.
That said, we never know for sure there aren't other parks. Evidence suggests, there isn't, all hosts we see are West themed, and Ford seems to be the only controller of the code. But I can think of at least 2 or 3 twists were they introduce a new park on season 2.
EDIT: As of the Season 1 finale there is ample evidence there is more than one park. The Samurai themed SW (SamuraiWorld?) we see, is a clear hint they are at least working on it. Then there is the note passed on to Maeve about her daughter, that points her location to "Park 1", which would necessarily mean there are more parks.
After the season finale we can safely make a new answer here:
There is almost certainly at least one other world that Ford and his team is working on with a Samuri theme, perhaps EastWorld. We see clear signs of this when Maeve is trying to escape and walks through the section of the complex were the Samuri are being worked on. They are already practicing complex behaviors so I think its safe to say that EastWorld is either in the final stages of production or already open to the public.