Why do Telvanni not use stiltstriders?
Is it just because they are all connected to the water and can use boats instead, or is the an explanation in the lore?
Because I love Stiltstriders and prefere them over boats anytime. =)

Telvanni are powerful mages for the most part,and can probably teleport to their destinations.

In lore its because the Hlaalu and Redoran control the strider trade and seeing how theres only two telvanni settlements on vvardenfell they are pretty much useless to the Telvanni who mostly live on islands.

Telvanni do not like to travel anyway, they prefer stay in their tower

Remember reading from somewhere while playing Morrowind, that Telvanni prefer teleport over other transport to avoid meeting with Imperials, outlanders, n'wah etc.

Sorcerers used sorcery, not mundane transport methods. I mean... they rarely use stairs yet alone those Striders that take ages to come to their destination...

Several reasons. The OP has already stated one: why use land transport when almost all Telvanni settlements lie on the shores of the sea? The only exceptions are Tel Vos, which uses the port at Vos, and Tel Uvirith, which is in a pretty idiotic place anyway. Then there's the fact that the Hlaalu control the strider network (dunno why they let the Redoran use it, though), and that there's always teleportation for those who are skilled in its use. Which is a lot of Telvanni.

I wouldn't think that another House could stop the Telvanni from using silt striders if they really wanted to. That explanation would have to be fleshed out a little more for me to buy it.

The teleportation explanation doesn't hold to just the Telvanni because in the other house towns/cities, those individuals and groups magically powerful enough would travel by teleportation as well. Its not like the Telvanni wizards have a monopoly on that. The question isn't why aren't there silt striders for the powerful Telvanni magi but why aren't there silt striders to the Telvanni settlements around the towers.

The geographical explanation is the only one of the three that makes a lot of sense and its the only one needed. Just examine a fully fleshed out map and its pretty clear why as others above have pointed out. Added to the island and seaside factors already mentioned, you got some nasty-ass Red Mountain territory to have to deal with, if one were to contemplate going from Vos to such places as Balmora or Aldruhn. With the lay of the land, the intense cliff racer population, the blight storms, and the length of the trip you have a terrible hurdle to cross -- too much for a regular business.

Its a funny thing with telvanni and transportation, because getting about while doing the telvanni missions can be a bit of a pain, especially when you roleplay a real telvanni - i.e - someone who wants to have nothing to do with the mages guild (meaning guild guides are out the window for possible transportation). That leaves boats, dunmer strongholds and mark and recall for quick travel, which in some circumstances, means there is no other way except walking (or water walking). I really like the Rise of House Telvanni mod and their very well done implementation of telvanni teleports (a system which allows you travel between any telvanni stronghold once you have completed all the associated mini-quests) - makes the life of a house telvanni member a much less gruelling existence, and also a much more believable one.

Don't forgot other Dunmer.

Including other Telvanni!
Theyre the worst kind of all!
Yeah I was just wondering, because the PGE mentions that the Telvanni tame and handle and sell most of the Siltstriders in Morrowind.