It's one of the textbook definitions of a recession from a first year college economics course. That is, it's a standard definition of a recession.

For example, to quote the IMF:

"Most commentators and analysts use, as a practical definition of recession, two consecutive quarters of decline in a country’s real (inflation adjusted) gross domestic product (GDP)—the value of all goods and services a country produces (see “Back to Basics,” F&D, December 2008). Although this definition is a useful rule of thumb, it has drawbacks." (https://www.imf.org/external/pubs/ft/fandd/2009/03/pdf/basic...)

David Sacks points this issue out rather well.

Because the people who don't like this textbook definition being used when it's inconvenient, haven't proposed a better definition. What definition would you use to describe a recession?

Because its the standard and long-since-accepted definition. That it makes the ruling party look bad is immaterial.