1. Almost nobody outside the weird countries of the British Commonwealth (i.e. all of them, except for Canada) uses "telly" to mean "TV". Especially not in the US, where "tellisini" is apparently de rigeur. So where'd we get "telly" instead of "tel'-schwa"?
2. The other obvious pronunciation -- there are at least three of us who have to this point assumed it was "tel'-schwa-seen" -- is not explained away. To my ears, it sounds nicer. It's shorter, and ends with a nice familiar "n" sound, rather than the distinctly furrin-sounding "i" sound. It also rhymes with (and has the same number of syllables as) "magazine", which everybody's familiar with. Or "melamine", say. Or even "vaccine".
Or do you propose we start pronouncing it "vaxini"?