The Sisters' limited armoury is one of the things that makes them unique.
Lascannons really go against the "cleansing heretics with purifying flame" ideal of the Sisters. Giving them these weapons just makes them into another SM chapter.
Our current cheapest long-range solution, autocannon Redemptionists, will probably disappear the next time an official Sisters army list is released.
Don't like it? Boohoo. Cry me a river, build a bridge, and get over it.
AX