There are one or two misconceptions about plants generally in your question. Some plants do flower in mid winter or very early spring, when it is still cold - snowdrops and aconites are good examples. Generally, the bloom time for permanent plants is for a period of between 2-6 or maybe 8 weeks, with the exception of summer bedding or tropical/sub tropical type plants such as Impatiens or Pelargonium.
Nemophila are hardy annual plants which seed themselves readily - their bloom time is early to mid spring (April/May) and they will stop flowering after about 4-6 weeks. They choose this time of year to flower because it is not yet too hot for them - they must flower and set seed before hot weather arrives, because they are woodland plants. They stop flowering because each flower, once pollinated, will begin to produce a seed pod; whilst we may appreciate and admire the flowers, their only true purpose is really to be pollinated, fertilised and reproduce via seed.
If you wish to see them in flower, I'm afraid you will need to go when they are expected to be in flower, in the same way that you might visit a Rhododendron garden or a bluebell wood in May in the UK - go in July, and there won't be a bluebell or rhododendron flower in sight.