Here's why:

* Viscous Lava: Felsic lavas, like rhyolite and dacite, are high in silica content. This makes them very thick and sticky, like honey or peanut butter. This viscosity is crucial for dome formation, as the lava doesn't flow easily and piles up instead.

* Felsic Composition: Felsic lavas are rich in silica, aluminum, potassium, and sodium. They have higher melting points than mafic lavas (like basalt), which contributes to their slow flow rate.

Why not other types of lava?

* Mafic Lava: Basalt, a mafic lava, is less viscous and flows more easily. It wouldn't pile up to form a dome. Instead, it would create flatter flows or shield volcanoes.

* Ultramafic Lava: Komatiite, an ultramafic lava, is extremely hot and fluid. It wouldn't form a dome, but rather vast, fast-moving flows.

In summary: The thick, sticky nature of felsic lava is what allows it to solidify and build up into the dome-like structures we see.