The primary reason battery technology hasn't improved significantly in so many years is because of the limitations of the materials involved. Batteries are essentially made of two electrodes (usually metal) separated by an electrolyte (usually a liquid or gel). The electrodes are connected by a conductor, and when the battery is in use, electrons flow from the negative electrode to the positive electrode through the electrolyte and conductor.
The materials used for the electrodes and electrolyte are crucial to the performance of the battery. The electrodes need to be made of materials that are not only electrically conductive, but also chemically stable and durable. The electrolyte needs to be a good conductor of ions (the charged particles that flow through the electrolyte during battery operation) and also needs to be stable and not reacts dangerously with the electrodes.
There are a number of different materials that have been tried for batteries over the years, but so far, there is no perfect combination that meets all the necessary criteria. Some materials are good at one thing but not so good at another. For example, lithium is a very good conductor of ions, but it is also very reactive and can cause safety issues. Researchers are constantly searching for new materials that might be better suited for use in batteries, but so far, nothing has been found that is significantly better than what is currently being used.
This is not to say that battery technology hasn't improved at all in recent years. Researchers have been able to make some modest improvements by using different materials or by changing the way the battery is designed. For example, lithium-ion batteries are now commonly used in laptops and cell phones, and they are significantly better than the lead-acid batteries that were used in the past. However, these improvements have been relatively small, and there is still a lot of room for improvement.
