Because 150 years ago the labour to keep the estate running was very cheap. Now that labour is expensive, it costs more in maintenance than the property is worth, unless it's highly productive land. Reminds me of the joke, how to make a thousand dollars: buy a million dollar boat.
On other side my guess is also that net labour productivity of land has dropped significantly. What I mean that same amount of farm land does not produce same amount of excess labour buying power. So even if productivity itself for farming has risen massively. The amount of labour that you can buy with produced production has plummeted.
