the 8th, 10th, and 11th Congressional Districts are in Northern Virginia, but the 9th District has been in southwestern Virginia for over a century
Source: US Geological Survey, Congressional Districts - 113th Congress
The two Senators serving in the Congress from Virginia represent the entire state. They campaign throughout Virginia every six years for election, and visit nearly all of the state's 95 counties and many of the cities in the process because all votes in the state count towards the election of a Senator - one vote in Halifax County is as good as one vote in Loudoun County. The same applies to races for Governor, Lieutenant Governor, and Attorney General every 4 years.
Candidates spend most of their time and money campaigning where voters are concentrated, so most efforts are concentrated in Northern Virginia, Richmond area, and Hampton Roads. Roanoke has television stations that reach much of southwestern Virginia, and Lynchburg TV stations reach rural areas in Southside, so brief visits to those cities generate valuable media coverage. In addition, every candidate for one of Virginia's 5 statewide offices (two senators, Governor, Lieutenant Governor, and Attorney General) will appear in the Appalachian Plateau counties occasionally to show their commitment to that region.
In contrast, each of the 11 members of the US House of Representatives represent less than 10% of the state's population. Every two years, they campaign in just their district.
You won't see candidates for the seat in the House of Representatives from Third Congressional District campaigning for votes in Charlottesville, or supporters of the candidates in the Ninth District handing out sample ballots at Metro stations in Arlington.
Candidates for all of the House of Representatives attend fund-raising events across the state (especially in Richmond and Northern Virginia), but the members of the House of Representatives have a narrower focus than members of the Senate. They represent their districts, not the entire state. It is smart politics to focus on issues of concern to their geographical region.
District boundaries shape the priorities of elected representatives. Odds are, in any Congress the elected member to the House of Representatives from the Sixth District (including much of the Shenandoah Valley) will care more about the budget and regulations from the US Department of Agriculture than the Congressman from the 10th District (including urbanized Arlington/Alexandria). The elected member to the House of Representatives from the Ninth District (including much of Southwestern Virginia) will normally care more about burley tobacco than the Representative from the 8th District (including Fairfax/Prince William suburbs).
Who in the state's delegation to the US Congress cares the most about Navy contracts for shipbuilding and repairs, or how many aircraft carriers will be based on the East Coast?
The 11 districts for the House geography of the districts affect campaign styles. In Northern Virginia, a candidate for the 8th, 10th, or 11th districts can visit all the precincts in that district easily (though they will have to fight the urban traffic jams). Candidates can purchase advertising on one radio station such as WTOP and reach the entire district with one "buy."
In contrast, the candidates in the 9th District have to drive for a half-day just to get from Cumberland Gap to Roanoke. Using radio and TV in his campaign requires more complex calculations regarding what stations reach which geographical areas, in addition to the standard assessments of what type of voter listens to country vs. Top 40 music...