This can't be answered without a biased view point. However a general idea is that Miku was aimed at Otaku's and was very successful in it's marketing of building a large audience. The hostility could come from the idea that new vocaloids will take attention from Miku and make the fandom become unstable and eventually forgotten.
You also have to look at the communities involved. In particular, Vocaloid related fandoms like Vocaloid Otaku are very open to both new UTAU and Vocaloids alike. Wheras some Crypton Future Media based fandoms are very closed to other Vocaloids. This, however, is because VO forums has a fairly experienced set of fans behind it who have worked with the software and know their ends and out and have attempted to understand what Vocaloid is. Many of the more closed fandoms tend to focus on the music more rather then the software.
There are some fans who believe that Crypton's Vocaloids are the only Vocalodis and dismiss all others. There are some who do not understand that Gumi is a Vocaloid, they presume by the box that "Megpoid" means she is a different software entirely.
These are some e of the many reasons why this question cannot fairly be answered without bias. There is simply too many considerations to take into account.