Why? One possibility is bad luck. January’s snow seems to have hampered exporters more than importers. And before then imports were boosted by the car scrappage scheme.
There might, though, be some deeper factors here.
The optimistic possibility is simply that it takes a long time - many years, maybe - for exporters to respond to a weak pound. First, they need to be confident that the lower pound is here to stay. Then they need to invest in marketing and working capital. It’s only then that exports grow; this sluggish response was brilliantly described by Paul Krugman in his book, Exchange Rate Instability.
The problem is, though, that the lags might be especially long now. When firms are unable to raise finance and/or are pessimistic about global demand, they are especially unlikely to invest in an export drive.
A second possibility is that exchange rate moves don’t raise export volumes much at all, as firms instead use them to raise profit margins. This could be because they price-to-market, so a UK exporter to the US keeps his dollar prices unchanged when sterling falls.
A third possibility is that exports have a high import content, with the result that rises in exports are necessarily accompanied by rising imports; since 1990, the correlation between annual growth in non-oil exports and in non-oil imports has been 0.82 (R-squared = 67%).
One reason for this is that exporters import raw materials and components. Another reason is that to export more, you need more workers, and workers buy imports: consumption bundles are inputs too.
In this context, we shouldn’t be surprised that, since troughing in Q2, the 10.5% rise in non-oil export volumes has been accompanied by a 6.2% rise in imports.
With imports greater than exports, this correlation makes it mathematically hard for net exports to contribute much to GDP. If export volumes were to rise 10% from Q4's level and imports 6%, it would add less than one percentage point to GDP. This means that if the economy is to grow at the 3%-plus rate which the Treasury expects over the next few years, the vast bulk of that growth will have to come from domestic demand.
* I might be doing Osborne a dis-service. In his Mais lecture, he says he wants a higher share of exports in GDP, not a higher share of net exports. Maybe he’s just aiming at a more open economy, rather than one in which net trade contributes heavily to GDP growth.