Because swali is a class 5/6 noun [plural maswali] and they take l and li- in concords (or nothing / j- / ji- on adjectives).

class 5/6:
swali lako jipya linatosha = your new question is enough
maswali yako mapya yanatosha = your new questions are enough

You might be mixing it up with class 9/10, which looks like this:

swichi yako mpya inatosha = your new (light) switch is enough
swichi zako mpya zinatosha = your new (light) switches are enough