That's quite a massive assumption. If we do a job search on startup-specific job boards, (like startuply.com and startuphire.com), you'll see the same sort of thing. On Statuply, "php" returns 280 hits, "rails" 121 and "django" 35. On StartUpHire, "php" 458, "rails" 119, "django" 9.

Long story short, don't believe every poll you read on the internet.

Also, don't take this as me talking bad about Django; it's a great framework and programming is not a popularity contest.