It's a very good question, and to my shame amid all my quantitative speculations it's something to which I'd never given much thought.

One problem is that there's no unanimity about what happened, or when or at what rate: some see the key changes in technological advance that's most marked in the 19th century, others date the start to 1750, still others to 1700, while I see prices, urbanisation and trade as indicating rising labour productivity from the 1650s (but with the fastest gain paradoxically from the 1870s to the 1900s when the classic growth period is over and arable output is in freefall). Some of course (notably the irrepressible Greg Clark) see no revolution at all.

One answer is that it did happen elsewhere. Britain borrowed innovations freely from Holland, and nearby districts of the Low Countries and northeastern France were noted for their performance (though France generally lagged behind). But Britain certainly took the process a good deal further, apparently doubling output per agricultural worker by 1850.

Instead of why, it may help to ask why then (though this of course brings us back to the issue of "when?"). An explanation may be offered by what appear to be exceptionally high land values in the 17th and 18th centuries, notably in the middle half when the general price level was falling. Assuming that land prices and rents reflect the capacity to realise a return from cultivation, the widening mismatch in the century to the 1740s seems suggestive of early productivity gains. But the already high cost of land in the first half of the 17th century (later higher still in real terms) may have been a driver of more intensive cultivation.

That leaves another "why?", and here my intuition runs out. It does seem a mystery, especially coming after the freeing up of the great monastic estates, a fresh batch of which had lately been dumped on the market as values were outstripping even the inflation of the Price Revolution. Part of the answer may lie in peculiar workings of money supply growth from the 1590s (though I don't see why that should have impacted disproportionately on land). Or perhaps it's a lasting consequence of the exceptional population growth of 1560-90, which could have had such an effect but one that should have been reversed by the contraction a century later. I still get the feeling I'm missing something.