If you write a piece that "plays well", a player can play it well — and give a more compelling performance. What makes a piece "play well" isn't necessarily that it be "easy"; more that it be natural and idiomatic for the instrument... which will in turn be easy.
If you write a piece that a player struggles to play, that will sometimes come through via a muddled and strained performance. There are famous exceptions that prove the rule on this, such as the extremely high register opening bassoon passage from Rite of Spring through which Stravinsky intended to convey strain, or the trumpet squeals of Ray Nance at the end of many Ellington arrangements.
Generally, though, you want the players to be able to play your piece well. And "err on the easy side" is an effective way to do that, especially when writing for instruments you aren't personally an expert with.
If you write a piece that a player struggles to play, that will sometimes come through via a muddled and strained performance. There are famous exceptions that prove the rule on this, such as the extremely high register opening bassoon passage from Rite of Spring through which Stravinsky intended to convey strain, or the trumpet squeals of Ray Nance at the end of many Ellington arrangements.
Generally, though, you want the players to be able to play your piece well. And "err on the easy side" is an effective way to do that, especially when writing for instruments you aren't personally an expert with.