Our tongue has specific areas for specific taste. Bitter tastes are tasted on the rearest area of our tongue, close to the throat. The reason why God decides to give us bitter senses nerve on the end of our tongue is His secret knowledge.
But it is our nature as human to questions and to conclude things based on our knowledges and experiences of things.
And the simplest of reasons -that comes from my own deduction- of why God put bitter senses on the rearest end of our tongue is so that we taste bitterness last.Imagine if you eat something that has a mix of sweet and bitter taste, some chocolates for example. If you taste the bitter taste first, due to the nature of human to despise bitter taste, you'll likely to spit the thing out, and you'll be unable to taste the delicacy of a chocolate (or any bitter-sweet mix). So it's a very blissful decision for us that God has design our tongues to be like so.It could also be some kind of a philosophical warning : sweet thing sometimes tasted first, and when they ends, bitterness comes. When things come to us in avalanche of sweetness, it is time for us to be aware of to prepare for the coming of bitterness. So when bitterness come, regrets don't come too much.And when bitterness is tasted, the way to purge it out tends to be painful.