I used to live at Kever Rashbi, so maybe I can speak about things that I’ve seen/heard/know.
It’s not a big deal for ALL frum Jews to go on Lag B’omer, but it’s a big deal for MANY here in Israel.
Most of the Jews that I’ve talked to, go simply because they WANT to go; it just so happens that their sect also is going. In other words, most people I talk to go to Kever Rashbi because they see it as a wonderful treat & not as a religious requirement that they must fulfill…many frum Israelis (ashkenazi & sephardi) love Rebbe Shimon intensely & they gladly will put up with the transportation issues in order to share in Rebbe Shimon’s simcha on his hillula.
As for how long have Israelis been going, I’m not sure. But in the time of the Arizal people were already making the trip. So, it’s been going on for a while.
By the way, I don’t know if you’ve ever been to Kever Rashbi, but their are massive crowds of people there every Shabbos & Chag. I point this out because some people think it’s only a yearly thing for Meron to have big crowds. But it’s not (okay, maybe not 400,000 or whatever people)….I also want to point out there are many other entire sects that come up there to daven, not just Breslov like some people think.