If we can use merge sort tree type thing to merge to hulls why do we need RMQ then why cant we use segment tree to answer the queries?
Because using segtree, we shall have to merge logN hulls (logC hulls if using sqrt dec). But by RMQ, we need to merge only two hulls (three hulls if sqrt dec, border block elements). Using segtree would increase per query complexity by log(N)