You had 2 opportunities in your hand out of which 1st was in the IT field and the other in your cour area and it is much understood that any engineer would like to work in his/her mainstream as it helps them to explore, learn and even put into practice what they have learned. Nowadays, we see many engineering pass-outs working in IT, Banking or other field but that is due to a good salary package or lack of opening in their branch.
As you were waiting for your core branch to provide you a chance of service but they never returned and the other chance is to hold on to the IT field due to the current scenario, you need to prepare well and expect the question of late reply to the application or selection. There are two options with you as other members have commented:
a) You tell them the truth but this may either go well with the interviewer or may back-fire on you or your selection. Many interviewers like the frankness of the candidate but as you are a fresher, I would not recommend you this as this may show your changing attitude and they may not like to take the risk of training you and if you get a better prospect, you may resign is what they may feel.
b) You need to find a solid reason with some supporting answers to the query. The interviewer may not be convinced at the first instance but you need to support your answer with some evidence or answer which can be justifiable and trustworthy. Never may they feel that you opted for the job on the secondary ground.
There are various reasons that can be attached like higher education, health issues ( it can be you or anyone in the family), Not in town, etc. Be sure that whatever reason you present, you need to be ready with some other questions related to it, so be prepared.
Now, prepare yourself for the interview with the IT company requirement like programing language, software or hardware, etc. Why opt for IT when you are from Mechanical branch, etc. Be confident, focused, well dressed i.e. presentable(Your first impression is the best impression), relax and listen to what they are asking, then reply in a soft, calm yet persuadable manner.
“The most important thing in life is to learn how to give out love, and to let it come in." — Morrie Schwartz