bravozulu
I was thinking it might be introduced from a more northern climate but this article has that happening and they produce leaves much earlier when it isn't as cold a winter. It may have been a very cold winter and ash are especially sensitive to that when it comes to budding time. The tree is probably adapted to using a more conservative strategy and not risking late frost kills. .

Malicai
well actually in april they have their buds ready to burst. But they always make sure the temperatures are right because if a frost comes the leaves will become dead and won't be able to flower. Another reason why they take so long is because they trees have thick bark and the buds may be to weak to break through it and wait until they are stronger to finally push through.