Here's why: that's where the Cubs played their 1918 World Series games. Why is that? The Tribune's James Cruisinberry explains, just after the Cubs clinched the National League pennant:
While the Cubs' management has already begun preparations for the big event, it is not yet determined whether the games will be played at Weeghman park or transferred to the home of the White Sox. Almost twice as many fans could be accomodated at the south side grounds, and if the demand for seats promises to be anywhere near the demand in normal years it is likely the games would be transferred from the north side. Business Manager Craighead of the Cubs stated that the place would be determined within a few days. If applications for seats are big, making it certain the north side park would not accomodate the patrons, undoubtedly the games will take place in Comiskey's park.
Remember this: the Cubs were playing just their third year on the North Side; there wasn't the attachment to the park now known as Wrigley, and it seated only about 20,000, while Comiskey Park was, at the time, capable of seating 32,000. The phrase "normal year", obviously, refers to the fact that World War I (the "Great War", as it was known at the time) was raging in late summer 1918, the reason the season was shortened and the World Series held in early September. The games were played in an unusual pattern: the first three in Chicago, the next three (and if it had gone the full seven, Game 7 as well would have been) in Boston.
Attendance for the three World Series games in Chicago, played on a Thursday, Friday and Saturday, was 19,274 for Game 1, 20,040 for Game 2 and 27,054 for Game 3; only the Game 3 crowd would have overflowed Weeghman Park.
As nearly as can be doped out of the tangle produced by the curtailed schedule, the Cubs drove the ultimate spike into the 1918 National League pennant yesterday when they trounced Brooklyn in both ends of their double header by scores of 8 to 3 and 3 to 1, respectively. The two edged victory gave the Mitchell tribe a lead of eleven and a half games over the Giants, and so far as plans are known the McGrawites cannot play enough games to catch up even if the Cubs lose the rest of their battles.
The season had been truncated due to the war, and there was in late August still some confusion over exactly when it was to end. In fact, there was some question about whether to play the World Series at all, though the teams -- there was not yet a centralized commissioner's office -- finally agreed to do so. (Also note the two-word spelling "double header" in the quote.)
Here's how the Tribune wrote up how to write in for tickets:
Business manager Craighead of the Cubs issued the following announcement yesterday regarding the sale of seats for the world's series games to be played in Chicago on Sept. 4, 5 and 6: Time of game: 2:30 p.m. Box and reserved seats will be sold for three games. Price of one box seat, three games, war tax included $9.90. Price of one reserved seat, three games, war tax included $4.95. To receive attention make all reservations in this manner: By mail only addressed to Walter Craighead, business manager Cubs ballpark, Clark and Addison streets, Chicago. Accompanied by money order, cashier check or certified check payable to Chicago National league ball club. Please do not send stamps, currency or personal check. Include self addressed, stamped envelope with special delivery postage, will assist in prompt service. Regular patrons having season reservations will also please obtain their tickets in the above manner. You are respectfully requested to make your reservation as soon as possible. General admission tickets on sale each day at ball park. Bleachers 55 cents, war tax included; pavilion $1.10, war tax included. For any additional information phone Wellington 6350.
How polite! (Also note: taxes included in the price.) I'm not sure why that said "do not send stamps", though I seem to recall seeing that kind of wording on similar types of things as late as the 1970s. The Cubs' somewhat freak pennant -- they wouldn't have as high a winning percentage until their next pennant year, 1929 -- came on the strength of good offense and pitching; the team led the league in both most runs scored and fewest runs allowed, and had there been such a thing as a Cy Young award that year, Hippo Vaughn almost certainly would have won it. Vaughn led the league in wins, ERA, ERA+, WHIP and shutouts.