[joenathanone]

I can think of three possibilities(there easily could be more)

1. There are contracts in place that cannot be broken.

2. Money comes first, then life and limb.

3. Restrictions on agriculture use will lead to higher food prices, and this is considered the lesser evil.

[jcranmer]

4. Farmers are historically a cossetted political class that politicians are rarely wont to cross.