Because 'Olympic Park' is the name of an official entity at an Olympic Games. Just like Olympic Games is the name of a competition.
And one can use indefinite articles before proper nouns that are names. I just did it with 'Olympic Games'. (See also a World Series example below.)
Note that you can also say Olympic park (i.e., a common noun) when referring to a park that happens to be an Olympic Park.
This is the same as referring to a country's National Zoo (the name of an official entity) as a 'national zoo'. The only difference is that 'Olympics' is always capitalized.
...an Olympic Park is typically a central campus for sports and ceremonies related to the games, but in Boston's case, Olympic Park would be all of downtown. [emphases mine]
Source: Boston is Olympic Park, Dorchester Avenue is Olympic Boulevard--UPDATE 2/4/15
The proposal does not include plans for an Olympic Park, O’Connell said. The city itself would fill that role.[emphasis mine]
(But note the very next sentence: “We see Boston as the Olympic park,” he said. And see below for my comments.)
Source: Lawmakers warm to Boston Olympics bid
The Coastal Cluster will include five ice sport venues, with the stadium for ceremonies centred in an Olympic Park [emphasis mine]
Source: Sochi 2014 Venues
All uses of Olympic Park are as names of an official entity, akin to National Zoo as the name of an official entity.
On the other hand, when referring to a park that happens to be an Olympic Park (the official entity), Olympic park can be used.
Here the capitalized adjective Olympic is used with the common noun park:
“We see Boston as the Olympic park,” he said.
Source: Lawmakers warm to Boston Olympics bid (ibid)
"If you really want to develop a part of a city like East London, it probably makes sense not to use 250 acres [100 hectares] and hundreds of millions of dollars on an Olympic park and stadium. There are probably better uses for that money," Zimbalist said. [emphasis mine]
Source: Pictures: Past Olympic Venues—Rotting, Renovated, Repurposed
This is equivalent to saying "let's not build a national zoo." One is referring to a zoo that happens to be a national zoo, but one is not referring to the official entity's name.
For a related example that shows the use of the indefinite article with the name of an "official entity," here a sports championship series:
World Series is always capitalized even when it does not refer to a specific one.
And one can use an indefinite article:
Remember that a World Series that features the New York Yankees is sure to draw more attention than a World Series that does not.