Plants vs. Zombies – Why is Survival Mode Not Appearing?

Since you are already familiar with the PC version, I'll go into the other ones:

iPhone version

It contains the entire campaign, as well as different achievements (also different Easter eggs, such as what happen if you scroll down the achievement listing continuously). The puzzles are mainly the same as well.

• Doesn't have survival mode.
• Doesn't come with most of the bonus games (including the plant garden, the tree of wisdom, as well as the Zombiequarium minigame)
  • These must be purchased with either in-game coins or with real money using microtransactions
  • All Mini-game packs can be purchased for 50,000 coins each, the I, Zombie game pack for 150,000 coins, the Vasebreaker game pack for 150,000 coins, and the Last Stand game pack for 100,000 coins.

iPad version

Identical to the iPhone version, except it now includes the Survival mode. However, now it includes a new way to improve the power of your cobb plants (by putting butter on enemies first). This is making use of the iPad multitouch mode of up to 11 touches at once (think launching multiple walnut in walnut bowling at once). Still no zen garden.

Read more on the Wiki here: Game versions of Plants vs. Zombies

I've searched fairly extensively and I can't find anything that suggests it gets harder.