Probably because she has a mobility spell in her kit and a huge amount of CC?
You can count Lissandra as probably the only champion in the game who legit pays for her mobility by having lower power in other places.
Xerath is more dominate than lissandra by range and CC, I'll explain how and why, Xerath has almost the exact same CC as Lissandra does because of his W, his W causes people's slow movement speed by 60 / 65 / 70 / 75 / 80% and he is free to abuse it every 10 seconds maybe less because of cool down items and thats not all, his W has a very long range that inflicts hard AP damage towards the enemy who is hit with it, hell he can slow an entire group with that W so you can't say Lissandra pays for her mobility when she's a short ranged mage that has to be in close contact in order to use her spells. Her E has an extremely high cool down for it not being an ult and the fact it isn't as long as people think, not only that you have to wait for a certain amount of second for her E to teleport her to a safe distance which most of the time people are able to catch her before she can escape.
Which champion is better is irrelevant to the topic at hand.
Lissandra has a mobility spell as well as a low cooldown slow and root. Neither Syndra nor Xerath have a mobility spell. The fact of the matter is Lissandra can both escape and engage with her E despite your claims to the contrary. Its pretty obvious why Riot would not want to give her higher base move speed. Namely because it would make her kiting ability even stronger and be oppressive towards melee champions.
The main reason that Lissandra is weakish right now particularly in mid lane has nothing to do with her move speed. It has to do with the fact that she has very low range spells and is easily bullied or poked out by longer range champions.
No that topic IS relevant because you mentioned about CC, you say she has a low cool down yet thats specifically her Q, her Q is a SKILL SHOT and is SHORT ranged, one miss with her Q can kill her if she's being chased that badly she DOES NOT have a low cool down for her root and her root is ALSO short ranged which again if she doesn't root anyone she's screwed. You shouldn't even say riot doesn't do this or that because we have other cancerous champions such as GNAR, KALISTA, AZIR, FIZZ, AKALI, ZED, SYNDRA, XERATH, the list goes on. Everyone has complained for these certain champions and if Lissandra was that much of a problem she would've been shown in the LCS or by twitch streamers which you don't see at all because even they say that her short range causes her death. Annie is short ranged but her benefit is how she can stun whenever she wants, she even burst with it, I always bring her up because shes similar to lissandra but annie is more helpful cuz of her passive and tibbers. It's true that Lissandra engages with her E + R + W + Q but its a SOFT CC it doesn't burst till she reaches very late game and her E is nothing but a slow teleportation that works half the time when she's in grave danger. It's never used for "damage" cuz it doesn't do any actual burst damage + its a skill shot as well. Even if its only +5 or +10 it can still benefit her in ways!