Why don't people play Galio more often?

Mana costs very high, largely flash dependant for his ult and his ult is easily qss'd or crucibled.

The two glaring issues he has are his nearly complete reliance on flash for ult positioning, and the abundance of things that can preemptively queue a spell (vayne condemn) or activate an on-next hit stun (Udyr bear stance) that will instantly cancel his ult.

This is actually all made worse if you try to build him as a tank, as a lot of people mistakenly (imo) will do. When the damage on his Q and E go down, so does his threat if the ult is on CD. Built as a mage, he actually merits some attention based on the raw damage off his Q and E. However, the late-game cooldowns on the Q and E tend to be longer, with the spells being a bit more difficult to hit, than those of his true mage counterparts.

Itemization sucks for Galio atm, his range is poor, and he's immobile

Because AD mids are FOTM right now, which totally defeats the purpose of picking Galio (anti-AP). Zed, for example, can just push with his free Q spam then roam bot for a double kill (because bot lane never wards and never pays attention to pings).