Every time I've seen an IPL game or spoken to Indian cricket fans, they pretty much know what is going on and the stadiums are packed.
Attendances for first class cricket all overt he world are dropping and from pics and videos I've sen of various matches, they are largely played in front of empty stands. I know, you'll probably post something about 20000 fans at some stadium at one time but they are the odd occurrence, not the rule. Often as a result of free ticket giveaways.
Now I'm sure most Indians love watching all formats of the game on their TV sets, as they do in Pakistan or Sri Lanka or the Windies but let's be honest, during the longer forms of the game, stadiums sit empty for the most part.
The following for tests is much more than IPL will ever have. The tracking of popularity based on how many people go to the ground is misleading.
There was a film released in India called 'Hamshakals', basically its worst movie one ever made. If aliens came to Earth and saw the film they will assume 'intelligent' life doesn't exist here. A million people probably still saw it thought in theatres. The Kolkata Test follow on test match which was jam packed on the final day, had about a hundred thousand people in attendance, much less than those who went to see 'Hamshakals'. If you go purely by 'more people turned out' then you will conclude, that people are more interested in 'Hamshakals' than they were in that test match.
IPL is good 3 hr outing with family, its short and mostly late in the evenings after work and hence people go to those games. You cannot expect the same reaction for tests. People in a developing economy like India don't have the time to spend a whole working day attending an entire days play, which may not even be conclusive for the game. Its just not going to happen. Perhaps in the early 2000s when the things were still slow economically, and work was chilled, people still used to go out in numbers. I still remember scenes from the Kolkata follow on test, and it was jam packed on the final day. Those days are done. Tests will still get good crowds on the weekends, but thats about it.
The difference is not about liking one format more than other, its more about the timings that T20 attendances are higher than tests, and the work culture that now exists in India. Earlier one could get a holiday to attend a test day, not anymore. Also leaves at work places are at a premium so no one wants to use one for a test match. Test matches are still hugely followed, and performances kept track of.
Also I don't know who you speak to about IPL. If you mean the hardcore cricket base like the one on this board, then naturally they would know as they follow cricket full time. However you speak to the avg fan, most of them don't follow IPL anywhere near as closely. If they tune in, then they tune in knowing there will be an IPL match on, but till they tune in they don't have a clue who is playing who. All they know is tht a match is on.
You can't even begin to compare the following for IPL with tests. When India are playing Tests (or ODIs), EVERY person you meet, will know what is going on, who scored how many runs/wickets etc. Even if people were working the whole day and missed the game and were not following it online they make it point to catch up on the highlights at night, or read up on the game, and know everything there is to know about an ongoing Test match.
With IPL, they can barely tell who is playing who. The only thing is now the way things are economically in India and work culture wise, tests will not get a full day's capacity crowd, any time soon.