Flirting is sorta no strings. It can be relatively easily forgotten, unless they've got issues, and, well, anyone too bogged down by baggage to not flip out at someone for flirting with them isn't really relationship material anyway, so you've found out without also having them think you were necessarily serious with the worse reaction that implies.
Flirting is asking someone to play. Declaring that you like someone to their face is demanding satisfaction in one form or another.
Also, it is physiologically and psychologically stressful to be in limbo like that and to drag out a decision like that for both parties. Far too likely that the stress and pressure would preclude actual weighing of things and result in a no of convenience that, for convenience and stress's sake, will be unlikely to be re-evaluated when a different tactic could have gotten a more fair consideration.
And, well, it's not exactly the kind of topic that can really be ignored with complete tact. Putting someone on the back burner and never replying to them is kinda rude. Especially if one sees them often. Especially if one is interacting with them the next day. Or the day after that, and so on exponentially until it's a slap in the face faux pas and then one would be relying on their lack of confidence or life taking them out of one's life to prevent them from calling one on one's BS and instead have them let it die.