I saw some comments earlier that the delay was probably due to trying to avoid triggering a special election yet.

Massachusetts

Interm appointment by Governor

Special Election must be held 145-160 days after vacancy occurs

Here is the interesting part.

2) If the vacancy occurs before a specified date preceding the regular primary (HI-60 days; MN-11 weeks; NJ-30 days; NY-59 days; VA-120 days), the election is held the following November; if the vacancy occurs within the specified period preceding the regular primary, the vacancy election is held at the second November election after the vacancy occurs.

http://www.ncsl.org/research/elections-and-campaigns/vacancies-in-the-united-states-senate.aspx