Where's Siri? Tech bloggers across the web demanded answers to that question when they learned last week that Apple's spunky voice assistant wouldn't be featured on the new iPad, which is already selling out in the lead-up to its Friday release. The iPhone 4S's virtual helper has been a central part of Apple's recent marketing push — even sparking lawsuits claiming that Siri's impressive responsiveness in commercials is misleading — and is widely viewed as the company's biggest innovation of the past 12 months. So why isn't Siri on the new iPad? Here, five theories:

1. WiFi-only iPads can't handle Siri
To work properly, Siri requires a constant connection to the internet. The iPhone 4S has that — either "you're connected to a local WiFi network, or you're slurping in 3G data from your carrier," says John Brownlee at Cult of Mac. The problem with the iPad is that Apple sells two versions: WiFi-only models and 4G-equipped models. Apple clearly doesn't want Siri to fail "catastrophically" for WiFi-only users who periodically have no connection. The company either wants Siri "to work for everyone, or no one." 

2. Apple's data centers aren't up to the task
Siri may be a victim of her own success, says Jason D. O'Grady at ZDNet. The latest version of iOS has Siri speaking several languages: English, British English, Australian English, Japanese, French, and German. And if "too many people simultaneously [start] asking Siri silly questions," Apple's data centers might be overtaxed, leading to slow and inaccurate responses from Siri. The last thing Apple wants is a "bad user experience" for new iPad buyers.

3. The iPad doesn't need Siri
"The iPad is primarily an entertainment device," says Bryan M. Wolf at App Advice — you're much more likely to watch a movie on it than you are to look up directions. The iPhone 4S, on the other hand, is about getting things done on the go. "For example, Siri is great at directing users to and from a location like a gas station or place to eat." On the iPad, this type of functionality is much less important.  

4. Apple wants to sell more iPhones
Apple has a simple message for consumers: "If you want Siri, buy an iPhone," says O'Grady. Apple makes a ton of "money on iPhone sales because it (famously) commands a kickback from carriers from every user's monthly bill." And keeping Siri as an iPhone exclusive gives customers yet another reason to keep feeding the iPhone cash cow.

5. Siri is broken
This may be a "contentious statement, but it's true," says Brownlee. Siri is "less intelligent and less useful than it was five months ago." Apple co-founder Steve Wozniak noticed Siri's answers have gotten more perplexing, too. "I used to ask, 'What are the prime numbers greater than 87?' and it would answer," says Wozniak. "Now instead of getting prime numbers, I get listings for prime rib or prime real estate." Why? Apple is likely devoting less processing power to each of Siri's questions now that demand for the iPhone 4S is so high. Just "imagine what would happen to the service with the crushing weight of 60 million new iPads heaped down on top of it."