Why is Olympus Mons the largest volcano in the whole solar system?
This is mostly due to the fact that Mars does not have plate tectonics. Therefore the plate stays above the hotspot without moving, allowing magma to rise and pile up at the same place for millions and millions of years. Above the Hawaii hotspot, the oceanic plate is moving, so volcanism tends to drift away with time (actually the volcanism happens at the exact same place from a mantle point a view, but its surface expression moves with the plate). It's why rocks of the Hawaiian-Emperor seamount chain are older in the West and younger in the East. This is true even at the scale of Hawaii island itself, where Kohala and Mauna Kea are extinct, while volcanism$-$or rather the plate$-$has shifted to Mauna Loa, Kīlauea and Lōʻihi.
Imagine if all the magma comprising these islands had piled up at the same place, it could have built a gigantic volcano like Olympus Mons! Well... not really. There is another parameter to account for: a theoretical limit to how high a mountain can possibly get, because of compressive strength of rock (or glacial erosion in some theories). See for instance answers in these questions:

How high can a mountain possibly get?
Why is Mauna Kea taller than the maximum height possible on Earth?

On Earth the limit is ~10 km. So even if magma kept piling up at the same place, the resulting mountain would start to laterally spread or collapse. But on Mars gravitational acceleration is lower, making the limit much higher.