Twilight sparkle is a very important pony
She has important royal duties: traveling around the world meeting with ambassadors from other nations, giving addresses, making decisions...She also has her friends.
Some things are more important than other things.

Rainbow Dash has watched herself and her friends grow, succeed, achieve everything they'd ever hoped to achieve. A successful farm, a career as a widely recognized designer, a career in the Wonderbolts, a way to care for even more animals, a beautiful family, a crown.
Sure they don't see each other as often, but the fact that they've achieved their dreams is the most important thing...right?