Why charging NiMH batteries on fast chargers, shorten their life? Is this only the heat build up in the batteries or there are other reasons?

There are two reasons: Heat and gas pressure stresses.
Charging NiMH is an exothermic process (so opposite to NiCd, where charging reaction cools them down), so a lot of heat is generated in the cells.
Plus the charging generates hydrogen, which has to dissolve into the metal. With fast charging the hydrogen is generated on the metal surface and it needs time to diffuse into the bulk of the electrode. This creates a lot of internal stress in the material, leading to earlier cracking and failure.

Plus practically all water based electrolyte cells are designed to balance the charge by mild overcharging. This overcharging creates extra pressure and heat in the cells, mainly whenthe charging rates are high. And both stress and heat are detrimental to the life.

The NiCd benefited from the main charging being endothermic, so once you design the cells with really minimum internal resistance, the bulk charging precools them down, creating a significant "thermal buffer" to absorb the overcharge energy before the temperature rises to anything detrimental. Plus this room allows rather simple temperature based cut off (either a direct temperature sensor, or indirectly via dV/dT, utilizing the negative temperature coefficient of the cell voltage).

The NiMH charging generates heat by itself, so actually taking away a big part of the thermal buffer, so usually ends up by driving the cells harder.

And there is another aspect that makes the NiMH to appear "less tolerant": The NiMH's use o have nearly 5x higher capacity for the same size (600mAh of NiCd vs 3000mAh of NiMH for AA size), so "abuse" (high currents, overcharging,...) tolerance which is in reality related to the cell ability to dissipate the heat looks as way lower "Ah factor" for NiMH. An example is a statement of "NiCd withstands 10% Ah permanent overcharging, while NiMH only barely 5%", when in real life the same AA sized cell has to deal with the same 60mA splitting the water that needs to be recovered.

Aren't only SLA batteries emits hydrogen?

SLA should not emit hydrogen to the outside with normal use, first the charging controller should limit the overcharging, then the remaining gas generation should be recovered by the internal structures (one of the electrodes being "oversized", so capable to catch the gasses generated from the overcharging and turning them back to water) but some minor residual emission is there, as the seals are not perfect and mainly the cell containers are not designed as pressure vessels.

Commercial NiMH and NiCd are made as sealed pressure containers, so their residual emission is true zero (unless the safety relieve activates after heavy overstress, but there we are talking about letting them die with the least surrounding damage, not normal operation), there all the gasses from overcharging are recovered back to water. The possibility of higher pressure operation lead to way more effective water recovery, so tolerance of virtually unlimited overcharging (the limit is practically only the capability of the cell body to dissipate the heat and to some extend the ability to transfer oxygen from overcharging positive electrode to the oversized negative, so about 60mA for standard AA size cells, in the 150mA ballpark in the special high overcharge rated "solar" types)

And for the hydrogen in NiMH: Hydrogen dissolved in one of the electrode metal is the main, principal, mode how the chemistry stores energy on that electrode. And the fact the hydrogen is able to move rather deep into the material is what gives it the extra capacity compare to e.g. the NiCd, where everything is stored just on the electrode surface, so within not much more than a molecule size thick layer. Even when the electrodes are made with huge surface (sintered powders,...), still it can not compete when you can reach deep into the volume of the electrode.