Why do the Congress often imposes a deadline for states to ratify a constitutional amendment?

Answer 1

Because of what happened with the 27th, proposed 1789, and not ratified until 1992 -- 203 years later.

Also, by limiting the deadline, they can insure that the amendment is only voted on by those currently in power, and thus preserve their power base during future elections.

Answer 2

So the issue can be disposed of one way or another. Either it's ratified by a certain date, or it's taken off the table.