Grocery store prices in the Washington are in 1980 increased more than in any other metropolitan area in the country, the Labor Department reported yesterday.

Shoppers in the nation's capital paid 14.4 percent more for food last month than they did in December 1979, an increase of more than one-third over the national average last year.

The average American's grocery bill rose 10.6 percent between December 1979 and 1980. The previous year the increase nationwide, including the Washington area, amounted to 9 1/2 percent.

Why did food prices increase here by half again as much in 1980 as they did in 1979?

Government officials were at a loss to cite any agricultural reasons affecting this region alone. Ernie Moore of Safeway Stores blamed much of the increase on recent local labor contracts which boosted wages and benefits for food haulers and handlers by about 12 percent.

Jack Cergol of the Food Marketing Institute suggested additional causes, including energy, transportation and Washingtonians' penchant for buying steak rather than hamburger.

Buffalo experienced the second-highest increase, 12.8 percent. Washington's increase far exceeded that of neighboring Baltimore, where grocery prices were up just 11.7 percent last month over the same period in 1979. Baltimore was tied for third with Dallas, followed by Kansas City, St. Louis, Detroit and Honolulu among major cities with 11 percent or higher increases.

According to Jesse Thomas of the Bureau of Labor Statistics, 1980 was the second-worst year for the capital's food prices in recent memory; in 1973, prices went up about 25 percent.

Increases in the area remained relatively stable at 1.9 percent through the first five months of 1980, then exploded in June when a 3.1 percent gain was registered. At the time, a BLS analyst explained that many stores were charging the same price for fruits in June as in May, although the new fruits were smaller. This caused the index to go up just as if there had been a price increase.

During the three summer months, food prices soared by 7.4 percent before subsiding in October. After a 1.1 percent rise in November, the December consumer price index for all urban consumers went back up to 1.6 percent in Washington. The comparable figure for the country as a whole was 0.7 percent, less than half the increase experienced here.

Like other regions, Washington was affected last month by sharp increases in the prices of vegetables, cola drinks and sugar. Higher price tags also were seen on eggs, milk, bread and apples. Locally, these were offset slightly by declines in the cost of beef, poultry, pork and oranges. So-called "other foods at home" showed a rise of 2.9 percent in December.

Nationwide the index for meat and fish increased somewhat, while that for pork and poultry registered "substantial" increases; beef was down almost everywhere.

Grocery store foods are priced each month. Food purchased in restaurants and alcoholic beverages are measured every other month in this area. On the national scale, the restaurant meals index show a 1 percent gain in December, while drinks were up 0.7 percent.