In this case, it is a shortened form of -(이)라서, which is a special case of the more common -아/어서. -아/어서 indicates 1) a reason (비를 맞아서 옷이 젖었다), or 2) first stage of a sequence (고기를 구워서 먹었다). (서 is often dropped, especially for #2, so you can say 비를 맞아 옷이 젖었다 or 고기를 구워 먹었다.) -아/어서 is usually changed to -라서 for the two special adjectives of 이다(is) and 아니다(is not). This change makes the phrase smoother and clearer for these frequently used constructs. And since they're not verbs, they can't have the #2 meaning and always signify a reason. So, - ... 후원하는 공연이다 + -아/어서 = 후원하는 공연이어서 (original) / 공연이라서 (slightly clearer). Another example: - A: 이것 좀 처리해 주시겠어요? = Would you please process this for me? - B: 죄송합니다. 제가 담당자가 아니라서 안되겠네요 = Sorry, but I can't, because I'm not in charge of it.

~~이라서 / ~이라 =because of / because