Why does dota pro matches have a small variety of heroes?

The meta develops over time as the event goes on. Heroes like techies got a pretty significant buff with their E now providing an aoe stun. Heroes that didn't get any significant buffs but are showing up in the meta during the event are probably because they fill a role to counter something in the enemies draft. For example, even though storm has not been given any major buffs in 7.33, midlanders that dive the backline have been very popular at the moment and storm fills that role very well.

As the event goes on, the pool of heroes will get larger. Usually groups have less heroes picked than main stage historically I believe.

There have actually been 104 out of 124 (84%) heroes picked in the Bali Major so far. If you only count those picked 10 times or more it's still 54/124 (44%).

Sure, it's not even close to a fully even representation, but still, it's not terrible. The last larger League of Legends tournament had 79 out of 163 (48%) picked at all, with 42/163 (28%) picked 4 times or more (less games in the tourney).

Apart from meta which has already been explained, players also play heroes which they're familiar with. So the saying goes - "I fear not the man who has practiced 10,000 kicks once, but I fear the man who has practiced one kick 10,000 times." Some players become synonymous with certain heroes because they become such dominant forces in the game when given those heroes. Pros will practice and get exceptionally good at a small pool of heroes rather than play all heroes at a mediocre level.

META

There heroes that are really safe to pick but if you compare with league of legends, dota actually have a big pool on competitive scene. I just think that 7.33 is a bad patch and there is too much overturned heroes and too much bad ones.

It's what is referred to as "the meta." As heroes get buffed, nerfed, changed, etc. the meta evolves. If Hero X and Hero Y are really good at doing something and Y gets a buff, maybe Y starts to get preferred. Now you know what was really bad vs X? Hero Z. Now hero Z's weaknesses aren't so obvious because Hero X just isn't strong enough to stand against all these heroes that are now being picked.

Eventually a couple months goes by and players have figured out the end of that X>Y>Z chain and a meta is born.

Now usually there are a few large tournaments each patch. During these tournaments we already know the strong heroes, but what about hero/team combinations? What about some cheese/pocket strat that a captain has cooked up? Sometimes certain players or teams just elevate a hero to broken status. It just works better than anywhere else. Well now the opponents have to decide "do i ban Y and Z and leave A in? Other team makes A just way too good, so ill leave Y and Z in play. Oh that worked really well, now how do I counter what else is being picked.

Eventually a new patch (more buffs, nerfs, changes) is released and everyone starts from 0 again. Sometimes (rarely) heroes get so overhauled they remove them from competitive play in order to get them somewhat balanced.

It's kind of late so I didn't do a great job explaining