why Clarksburg, Maryland doesn't have a town center

Not enough population. See "After nearly 20 years, future of Clarksburg Town Center plan may be at a crossroads" from the Washington Post. From the article:

Residents — the approximately 2,300 in the Town Center and 11,000 in surrounding Clarksburg developments — are left to do much of their shopping in Damascus, Urbana or Germantown. The rail or bus transit integral to the 1994 master plan is years away, if it comes at all. Rush-hour traffic on I-270 and Route 355 can be mind-bending.

If there were a market for the retail, it would already have been built.  Sector and neighborhood plans typically call for more retail than is supportable from the local population.  And by not providing more information about how much population you need to support the retail you want, plans typically under-provide for residential housing.

In work I quote a lot (summarized in "The long term shake out of local retailers and independent commercial districts"), from the late 1990s, it says that you need 30,000 people to support a typically sized neighborhood center of 50,000 s.f.

Now, with the rise in Internet commerce, both in terms of the destruction of various retail categories--meaning fewer stores to open--and in terms of a shift of 10 to 15% commerce to online options (from prescriptions to books to food) it's that much harder to support neighborhood/district shopping centers.

I'd argue that the number of people needed to support 50,000 s.f. of retail is probably closer to 40,000 people, plus the retail mix shifts more to restaurants, and most retail categories are in regional shopping centers of various sorts, and restaurants rely on high income consumers especially younger people who go out more/don't know how to cook.