In the first section on Browser Identification, I hinted at the reason - when the internet gets busy, even the simplest code goes astray. Despite it appearing easier to combine the code, the number of "else if" statements can become very daunting and very confusing. Nesting if statements that deep can cause errors. By splitting the code, I get two main benefits:
Don't take my word for it though. If you want to combine the code, try it and let me know how you get on.