Because of the lack of vectors? What do you prefer to use?
Its the lack of vectors mainly. If you produce a logo in Photoshop you would need to create the logo in many different sizes for various different applications or work on one huge file and reduce it down for every application as you cannot scale up without the logo pixelating. Using spot colours in Photoshop is also a bit messy and just adds complications to the process.
Using a vector drawing app like Illustrator you can create one file with extremely small file size and scale to suit any application without worrying about loss of quality.
As an aside, with JPEGs every time you open one and save it, it is compressed a little bit more, and over time the logo will look worse and worse. Thats why occasionally you get a letterhead or see a website with terrible pixelated logos. Use TIFs wherever you can when you need to use bitmap imagery i.e. photos. as they are uncompressed and use vectors for graphical elements like logos, symbols and icons etc.